Background In recent years gut microbiota has been found to play an important role in the occurrence and development of various chronic diseases, and diet is an important factor influencing gut microbiota. However, the effects of maternal high-fat diet in pre-pregnancy and pregnancy-and-lactation periods on offspring gut microbiota are still unclear. Objective To investigate the effects of maternal high-fat diet in pre-pregnancy and pregnancy-and-lactation periods on gut microbiota of offspring mice. Methods C57BL/6J female mice were divided into four groups according to the diet patterns (high-fat diet, HFD; control diet, CD) given before and after conception, namely the pre-pregnancy control diet and post-pregnancy control diet group (CD-CD group), the pre-pregnancy control diet and post-pregnancy high-fat diet group (CD-HFD group), the pre-pregnancy high-fat diet and post-pregnancy control diet group (HFD-CD group), and the pre-pregnancy high-fact diet and post-pregnancy high-fat diet group (HFD-HFD group). Female mice were conceived in the same cage with male mice after 6 weeks of feeding, and the successfully conceived females continued to be randomly divided into two groups receiving either high-fat or control diet, and when the offspring mice were born, they were breastfed directly by the mothers, with each mother nursing only one offspring mouse. The number of offspring mice in each group was 6, with half males and half females. The body weight of offspring mice were recorded and body weight gain was compared between the four groups. After the lactational period, fresh feces of the offspring were collected, and the fecal DNA was extracted. Specific primers were designed according to the bacterial 16S rDNA(V3+V4) sequence and then the sequencing was performed using the Illumina HiSeq 2500 platform. Species annotation and operational taxonomic unit (OTU) analysis of sequencing data were conducted using QIMME, USEARCH and R software. In alpha diversity analysis, ACE and Chao1 indices were used to evaluate species richness, Shannon and Simpson indices considered both species richness and evenness. In beta diversity analysis, principal coordinates analysis (PCoA) and analysis of similarities (Anosim analyses) were used to find the differences in composition of gut microbiota between four groups, and line discriminant analysis effect size (LefSe) was conducted to identify which specific taxa contributed to the significant differences between groups. Results A greater effect of post-pregnancy diet on offspring body weight was observed, and the lowest body weight was recorded in the HFD-CD group during the whole experimental period. The results of OTU analysis showed that high-fat diet during post-pregnancy period reduced the number of OTUs in offspring mice, and the results of alpha diversity analysis showed that high-fat diet during post-pregnancy period reduced the richness of intestinal flora (ACE, P<0.05; Chao1, P<0.05), whereas differences in the α-diversity indices did not show statistical significance in the offspring mice with pre-pregnancy high-fat diet. The high-fat diet at different periods also led to changes in the dominant intestinal flora of the offspring. The high-fat diet during post-pregnancy period increased the abundance of Tenericutes (P<0.05), and decreased the abundance of Bacteroides, Epsilonbacteraeota, Cyanobacteria, and Deferribacteres (all Ps<0.05). At the genus level, high-fat diet during both pre-pregnancy and post-pregnancy periods decreased the abundance of Lactobacillus (P<0.05), and high-fat diet during pre-pregnancy period increased the abundance of Alistipes (P<0.05), while high-fat diet during post-pregnancy period increased the abundance of Lachnospira and Ruminococcus, and decreased the abundance of Muribaculaceae and Helicobacter (all Ps<0.05). The results of beta diversity analysis showed that the CD-CD group had a similar flora composition to the HFD-CD group, and the CD-HFD group had a similar flora composition to the HFD-HFD group, and the results of Anosim analysis showed statistically significant differences between groups (R=0.743, P<0.01). The LEfSe analysis counted all species with an effect on the differences between groups greater than the set value, which were Lactobacillus in the CD-CD group, Clostridiales in the CD-HFD group, Bacteroidetes and Helicobacters in the HFD-CD group, and Blautia, Ruminococcaceae, and Roseburia in the HFD-HFD group. Conclusion It is found that varied effects of high-fat diet in different periods on the flora of the offspring mice. The high-fat diet during pre-pregnancy and post-pregnancy periods could reduce the abundance of Lactobacillus, but show different effects on the abundance of other intestinal flora such as Muribaculaceae, Lachnospiraceae, and Helicobacter differed. Diet during post-pregnancy period has a greater influence on modeling the offspring gut microbiota.