Objective: To understand the prevalence of group sex in men who have sex with men (MSM) and related factors in Taizhou, Zhejiang province. Methods: From March 2013 to September 2015, a cross-sectional survey was conducted among MSM recruited from gay bars or bath houses in Taizhou by using questionnaire to collect the information about their sexual behavior and HIV test. Results: A total of 1 435 MSM were recruited, 179 of them (12.5%) were HIV infected, 231 (16.1%) reported group sex during the past year. Compared with MSM without group sex, more MSM with group sex had more than ten male sexual partners (53.7% vs. 19.0%, χ(2)=125.527, P<0.01), had both male and female sexual partners (86.6% vs. 68.2%, χ(2)=31.935, P<0.01), used no condom at sex with female partners during the past year (28.1% vs. 19.8%, χ(2)=6.540, P<0.05). After adjusted for educational level and occupation, group sex was associated with age[≥50 years old vs. ≤24 years old, adjusted odds ratio (aOR)=2.25, P=0.027], marriage with female (divorced or widowedvs. unmarried, aOR=0.29, P=0.001), census registering (local resident vs. non-local resident, aOR=1.56, P=0.023), number of male seuxal partners (≥21 vs. 2-5, aOR=8.06, P<0.001; 11-20 vs. 2-5, aOR=3.25, P<0.001) and number of female seuxal partners (≥6 vs. 0, aOR=2.63, P=0.005; 2-5 vs. 0, aOR=2.58, P=0.001; 1 vs. 0, aOR=2.01, P=0.010). Conclusion: The prevalence of group sex in MSM was high in Taizhou, and their poor condom use and complex sex behaviors would result in higher HIV infection rate and risk of transmission.