Female sex workers (FSWs) in China are exposed to multiple work-related harms that increase HIV vulnerability. Using mixed-methods, we explored the social-ecological aspects of sexual risk among 348 FSWs in Beijing. Sex-work harms were assessed by property stolen, being underpaid or not paid at all, verbal and sexual abuse, forced drinking; and forced sex more than once. The majority (90%) reported at least one type of harm, 38% received harm protection from 'mommies' (i.e., managers) and 32% reported unprotected sex with clients. In multivariate models, unprotected sex was significantly associated with longer involvement in sex work, greater exposure to harms, and no protection from mommies. Mommies' protection moderated the effect of sex-work harms on unprotected sex with clients. Our ethnography indicated that mommies played a core role in sex-work networks. Such networks provide a basis for social capital; they are not only profitable economically, but also protect FSWs from sex-work harms. Effective HIV prevention interventions for FSWs in China must address the occupational safety and health of FSWs by facilitating social capital and protection agency (e.g., mommies) in the sex-work industry.