Research has demonstrated the association between social constraints and posttraumatic stress disorder (PTSD) symptoms among breast cancer survivors. Although perceived social support can buffer stress and improve emotional well-being, little is known about which type of social support is most effective in buffering the negative effects of social constraints among cancer survivors. We investigated the moderation of four types of social support (i.e., positive interaction, tangible support, emotional/informational support and affectionate support) on the association between social constraints and PTSD symptoms among Chinese American breast cancer survivors. One hundred and thirty-six Chinese American breast cancer survivors completed questionnaires that assessed social constraints, PTSD symptoms and perceived social support. Results of hierarchical regression analysis indicated that only support of positive interaction exerted a buffering effect, with social constraints associated with greater PTSD severity among survivors with low but not high levels of support of positive interaction. In contrast, high levels of tangible support potentiated the association between social constraints and PTSD symptoms. There were no moderating effects of emotional/informational support and affectionate support. These results demonstrated the roles of different types of social support in moderating the stress imposed by social constraints. Our findings highlight the importance of recognizing the potential cultural sensitivity of ethnic minorities in PTSD intervention and mental health services for cancer patients.