Although Social Network Site (SNS) usage has been shown to be related to online compulsive buying among women, little is known about the mediating mechanisms underlying this association. Based on the Interaction of Person-Affect-Cognition-Execution model (I-PACE) for addictive behaviors and social comparison theory, the present study examined the mediating roles of upward social comparison and state anxiety in the link between passive SNS usage and online compulsive buying among female undergraduate students. A sample of 799 Chinese female undergraduate students (mean age = 19.86 years, SD = 1.63) were recruited to complete questionnaires measuring passive SNS usage, upward social comparison on SNS, state anxiety, and online compulsive buying. After controlling for online shopping experience, the results showed that passive SNS usage was positively associated with online compulsive buying; upward social comparison and state anxiety partially mediated this link, which contained three mediating pathways – the separate mediating effect of upward social comparison and state anxiety, and the sequential mediating effect of upward social comparison and state anxiety. These findings can advance our understanding of how passive SNS usage is related to online compulsive buying among female undergraduate students. Limitations and implications of this study are discussed.