Introduction Nonsuicidal self-injury (NSSI) is a highly prevalent and serious public health concern among Chinese LGB (lesbian, gay, and bisexual) populations, and a variety of cognitive and affective factors caused by sexual minority stress is influential in the development of NSSI. This study tested a chain mediating model to examine the roles of negative body image (i.e., a cognitive factor) and anxiety (i.e., an emotional factor) in the relationship between maladaptive perfectionism and NSSI. Potential gender differences on these associations were also tested. Methods A total of 682 Chinese LGB individuals (57.8% gay men, 6.0% bisexual men, 16.0% lesbian women, and 20.2% bisexual women; mean age = 24.4 years, SD = 6.5) from across the country participated in this study in August 2020. They completed self-report measures of maladaptive perfectionism, negative body image, anxiety, and NSSI. Results There were significant positive correlations among maladaptive perfectionism, negative body image, anxiety, and NSSI in Chinese LGB populations. In the chain mediation analyses, maladaptive perfectionism was indirectly related to NSSI via negative body image and anxiety. In addition, negative body image was related to anxiety only in gay males, but not in sexual minority females, and anxiety was related to NSSI more strongly in sexual minority females than in gay males. Conclusions Findings of the study contribute to our understanding of the mechanisms of NSSI in Chinese sexual minorities. Maladaptive perfectionism was related to NSSI through negative body image and anxiety. Gender differences were noteworthy. Policy Implications Psychological education regarding proper body image expectations may be necessary for LGB populations, and clinicians may pay special attention to gender differences when addressing related mental health problems of LGB individuals in China.