Objective: To investigate the causal relationship between gut microbiota and constipation using Mendelian randomization (MR) method. Methods: Genetic variation data of gut microbiota were obtained from the MiBioGen Consortium database. The genetic variation data of constipation were sourced from the IEU Open GWAS database. A forward MR analysis was performed using the inverse-variance weighted (IVW) method with 2 511 SNPs associated with gut microbiota as instrumental variables, and constipation as study outcome, and a reverse MR analysis was performed with 13 microbiota-associated SNPs as instrumental variables and gut microbiota as study outcome. The heterogeneity was assessed using the Cochran test, reverse causation of SNP were examined using MR Steiger test, and the horizontal pleiotropy was assessed using the MR-PRESSO test and MR-Egger regression. In addition, the robustness of the results was verified with the leave-one-out. Results: Forward MR analysis results showed that an increased abundance of genus Coprococcus1 driven by host genetics was associated with a decreased risk of constipation (OR=0.791, 95%CI: 0.709-0.884), and increased abundance of phylum Bacteroidetes driven by host genetics was associated with an increased risk of constipation (OR=1.240, 95%CI: 1.102-1.394). Cochran test detected no heterogeneity (both P>0.05), MR Steiger test was not revealed reverse causation of SNP, and neither the MR-PRESSO test nor the MR-Egger regression revealed horizontal pleiotropy of instrumental variables (all P>0.05), and the leave-one-out method confirmed the robustness of results. Reverse MR analysis showed no association between gut microbiota and constipation (both P>0.05). Conclusion Genus Coprococcus1 and phylum Bacteroidetes in the gut microbiota are associated with constipation.