Objective: To investigate sexual behavior and related factors among middle school students in Chengdu,and to provide a reference for improving the content and the way of their sexual health education. Methods: Stratified cluster sampling was used to select 935 students from 11 schools in Chengdu. A self-designed questionnaire was implemented to estimate sexual behavior and related factors. Results: Thirty-six (3.9%) students reported sexual behavior during the past year. The difference between boys (5.6%) and girls (1.7%) was of statistical significance(χ2=9.79，P<0.01). 77.8% (28/36) of all the participants reported first sexual contact experience, 2.4% (22/935) reported homosexual behavior. Five out of 935 students had experienced unplanned pregnancy, four of them chose to have an abortion. Multivariate Logistic regression showed that masturbation experience, talking about sex-related things with friends, asking for help from friends when in sex-related troubles positively with higher risk of sextual behavior(OR=6.67,0.30,0.28,P<0.05). Conclusion There are multiple types of unsafe sexual behavior among middle school students in Chengdu. Strategies including positive behavior support, peer education, sustainable environment and youth-friend services are essential for health education in the future.