Objective：To analyze the high risk behaviors of HIV/AIDS and its influencing factors in men who have sex with man (MSM) in a city, so as to provide evidence for the targeted intervention. Methods: Between July 2006 and October 2006, 1000 MSM were sampled by snowball sampling and had a face to face interview by well trained investigators with informed consent. Questions on age, education background, and sex orientation were designated and 3 ml blood samples were obtained for examination of HIV and syphilis. χ2 test and logistic regression analysis were used for statistical analysis. Results: The average age of the 1 000 MSM was 27.8±9.1 years old. In the past 6 months, 43.6% of them had 3 or more male sex partners, 53.6% had unprotected anal intercourse with men, 8.0% had commercial sex with men, and 24.3% had sex with women. Elder subjects had a higher ratio of unprotected anal intercourse and malefemale sex; subjects with lower educational background had more sex partners and malefemale sex. Subjects working in the bathhouse had a higher ratio of commercial sex and malefemale sex. Subjects with a malefemale sex orientation or undetermined sex orientation had a higher ratio of commercial sex. Syphilis was detected in 93(9.3%) subjects and HIV was detected in 104 (10.4%) subjects; among them 18 subjects had both syphilis and HIV. Multivariate logistic regression analysis showed that the aged (OR=3.5, 95％CI 1.77.1), the number of male sex partners (OR=3.8,95%CI 1.97.6), the behavior of unprotected anal intercourse (OR=1.9,95%CI 1.03.5) and the infection of syphilis (OR=2.1,95%CI 1.43.4) were independently associated with the risk of HIV/AIDS prevalence; the knowledge of AIDS was the protective factor of HIV/AIDS prevalence（OR=0.5,95%CI 0.30.7）. Conclusion: High risk behaviors of HIV exist in MSM population in the researched city; there is local epidemic of HIV. Interventions should be given according to age, educational background, sex orientation, etc.