Previous studies have documented that parent-child attachment is linked to the mental health of college students. However, little is known about the mediating and moderating mechanisms underlying this relation. Based on attachment theory, this study examined whether security mediates the relationship between parent-child attachment and mental health among college students and whether problematic smartphone use moderates the second half of the mediation process. In total, 77,211 Chinese students (mean age = 20.09 years, SD = 1.37) were recruited from multiple universities. These students provided self-reported data on demographic variables, parent-child attachment, security, problematic smartphone use, depression, anxiety and resilience. After controlling for sex, age, paternal education level, maternal education level and family economic status, mediation analysis indicated that security mediates the association between parent-child attachment and mental health among college students. Moderated mediation analysis further demonstrated that the association between security and mental health is moderated by problematic smartphone use. Specifically, the relationship between security and mental health among college students was stronger for those with highly problematic smartphone use. The current study highlights the significance of identifying the mechanisms that moderate the mediated paths between parent-child attachment and mental health and has important implications for the prevention and intervention of mental health problems among college students.