Abstract: Objective To study the numbers of occasional homosexual partners of men who have sex with men(MSM)and to analyze its influence factors.Methods A follow-up survey was conducted among 405 MSM recruited for an open study cohort established in Mianyang city in 2009 using snowball sampling.A survey on related behavior and serological detection were conducted in the MSM.Results For the MSM,83.7%(339/405)had occasional sexual acts.The human immunodeficiency virus(HIV)infection rate was 7.7%(26/339)and the newly infected rate was 5.8/100(15/258)person years in the MSM.For the MSM,63.7% had an accumulative number of occasional sex partner of less than 10,20.4% had the number of 10-20,and only 4.4% had the number of greater than 90.The rate of occasional sex act was 100%(15/15),94.6%(53/56),81.3%(13/16),and 81.1%(249/307)for the MSM looking for sex partner in bar/bothroom,in park,in family,and through internet,respectively(χ²=15.315,P=0.004).The rate of using condom in every sex during previous 6 months was 55.0%(127/231),50.9%(119/234),2.9%(10/275),and 27.1%(19/70)for the MSM with insertive anal sex,receptive anal sex,oral sex,and vagina sex,respectively.The results of multivariate analysis showed that the independent influence factors for the number of occational sex partner were age,perceived risk of HIV infection,receiving education of the peer within a year,the number of cities for the sex partner living in,mainly having sex with occasional partner,and having multiple sex partner during the period with a regular partner.Conclusion Occasional sexual act is very common among MSM and with poor protection.There are relationships between the number of occasional homosexual act and demographic characteristics,sexual behavior,risk awareness,and social network among MSM.