BACKGROUND: In China, while the overall HIV prevalence has been decreasing within key populations, the epidemic among men who have sex with men (MSM) is still on the rise. This study aims to assess the HIV incidence rate and identify driving forces of HIV seroconversion among MSM in a closed cohort. METHODS: This study is a secondary analysis of a large trial of HIV testing promotion among Chinese MSM in 2016-2017. Sexual behaviors, HIV testing activities, and HIV serostatus were measured at baseline and follow-up every 3 months. HIV seroconversion in this study was defined as a self-reported HIV-positive test result. Participants who reported testing for HIV at least twice during different follow-up periods were included. Subgroup analysis and Cox regression were used to examine the correlates with HIV seroconversion. RESULTS: Overall, 347 participants were included in this study, with a mean age of 25.3 ± 6.1, and 71.2% were migrants. The sociodemographic characteristics of the included participants were similar to the rest of the participants in the trial (n = 1034); 7.2% (25/347) of participants seroconverted during the study period, resulting in an incidence rate of 15.56/100 person-years. In subgroup analysis, the HIV incidence rate was higher among migrants than nonmigrants (incidence rate ratio, 1.26; 95% confidence interval [CI], 0.47-3.87). In the time-dependent Cox regression model, bisexual MSM had a higher risk of contracting HIV than gay men (adjusted hazard ratio, 2.19; 95% CI, 1.02-4.72). CONCLUSIONS: Our findings suggest a high HIV incidence rate among Chinese MSM. Further expansion of pre-exposure prophylaxis and other effective HIV prevention interventions are urgently needed.