Phubbing harms relationships and reduces well-being of phubbees. However, limited research has explored the potential causes and underlying mechanisms for phubbing; and an important risk factor, social anxiety, has been largely ignored from previous studies. The present study employed a moderated mediation model to examine whether social anxiety was positively related to phubbing, whether problematic social networking mediated the relationship, and whether the first path of this mediating process was moderated by family socioeconomic status. The data of 1379 undergraduates (M[subscript age] = 18.84; SD = 0.94) were collected anonymously to test the model. Findings demonstrated that social anxiety was positively linked to phubbing, problematic social networking mediated the linkage, and family socioeconomic status played a moderating role in the first stage. Specifically, the indirect association between social anxiety and phubbing via problematic social networking was stronger for undergraduates with lower family socioeconomic status. Results provide a new perspective for understanding phubbing and highlight the significance of uncovering the underlying mechanisms.