Objective: To analyze the effects of 12week highintensity fitness exercise on body composition,lipid metabolism and gut microbiota in obese adolescents, so as to provide references for improving the health levels of obese adolescents. Methods: From January to June 2023, 20 obese adolescents from Huaifeng Vocational and Technical School in Huaian City were recruited for the study. Participants were assigned to an exercise group (n=10) and a control group (n=10) for a 12week exercise intervention by random number table method, and both groups had the same diet during the intervention period. The exercise group engaged in three exercises every week, mainly consisting of moderate to highintensity aerobic exercise combined with highintensity intervals. In the first week, there was a 30 minutes of aerobic exercise, followed by 10 minutes of highintensity interval training in the total intervention time each week, and the rest of the time was aerobic exercise with a total intervention time of 60 minutes to maintain; the control group did not receive specific interventions. Body composition was measured using bioelectrical impedance analysis, and lipid levels were determined using an automatic biochemical analyzer. The expression levels of serum inflammatory factors were measured at baseline and after 12 weeks of intervention, and gut microbiota was analyzed using 16S rRNA gene sequencing. Statistical analysis was performed using t test and Chisquare test. Results: After 12 weeks of intervention, the levels of triglycerides (TG), and lowdensity lipoprotein cholesterol (LDL-C) in obese adolescents in the exercise group decreased from (1.7±0.6, 3.5±0.8) mmol/L to (0.9±0.3, 2.6±0.4) mmol/L, while highdensity lipoprotein cholesterol (HDL-C) increased from (1.1±0.2) mmol/L to (1.4±0.2) mmol/L; and serum interleukin-1 receptor antagonist(IL-1Rn) decreased from (8.4±1.6) to (4.5±0.4) ng/mL in the exercise group (t=7.34,2.49,-3.05,2.56, P<0.05). The α-diversity results showed that the Chao index (268.00±22.67) and Ace index (243.98±38.64) in the exercise group were higher than those in the control group (184.52±19.28, 171.43±23.33), and the differences were statistically significant (t=2.48, 2.53, P<0.05). The Shannon index (5.36±1.41) in the exercise group was higher than that in the control group (4.73±1.12), and the Simpson index (0.78±0.10) was lower than that in the control group (0.89±0.10), but the differences were not statistically significant (t=1.83, -2.10, P>0.05). The β-diversity results showed that the intergroup differences in gut microbiota structure between the exercise group and the control group were greater than the intragroup differences, and the differences in gut microbiota structure between the exercise group and the control group were statistically significant (R2=0.083，P<0.05). After intervention, there were significant differences in the relative abundances at the levels of phylum, class, genus, and species in gut microbiota among obese adolescents between the exercise group and the control group (P<0.05). Conclusion The 12week highintensity fitness exercise can alleviate obesity symptoms in obese adolescents through the gut microbiota-lipid metabolism pathway and improve mild chronic inflammatory status.