The prevalence of HIV among men who sex with men (MSM)has been increasing sharply in China. The prevalence of unprotected anal intercourse (UAI)among them is very high, especially when male regular sex partners are involved. Many MSM having regular sex partners are engaged in multiple sex partnerships, which often involved UAI. This study interviewed 168 MSM in Nanjing, China, having had anal intercourse with at least one male regular sex partner in the last six months. Amongst the participants, 52.4% reported having had UAI with male regular sex partners whilst 50.0% reported multiple male sex partnerships in the last six months. Multivariate analysis showed that monthly personal income higher than 2000 RMB (multivariate OR = 0.49, 95% CI: 0.26-0.96), HIV antibody testing in the last year (multivariate OR = 0.19, 95% CI: 0.06-0.60), sexual position being both insertive and receptive during anal sex with male sex partners in general (multivariate OR = 0.43, 95% CI: 0.18-0.99; reference group: with receptive position only) were associated with lower likelihoods of UAI. Syphilis-related perceptions, including knowledge, cognitions (perceived susceptibility, perceived severity, self-efficacy in avoiding syphilis infection and perceived infectivity and perceived syphilis-related norms among MSM in general)were not associated with risk behaviors (UAI and multiple male sex partnerships). Our sampled MSM are exposed to very high risk of HIV/STD transmission. We found no association between syphilis-related perceptions and the two types of risk behaviors. Future studies are warranted to understand such associations in the context of sexual intercourse with non-regular sex partners among MSM.