There is evidence that grandchildren and grandparents in Chinese left-behind families (CLBF) suffer from immense life stressors. The well-being of both generations is of great concern. Guided by the family adjustment and adaptation response model and ecological model of well-being, the present study aimed to investigate the dyadic associations between family resilience and subjective well-being (SWB) of left-behind grandchildren and grandparents. Furthermore, we explored the extent to which relationship quality accounts for such reciprocal associations. A sample of 130 left-behind grandparent–grandchild (GP–GC) pairs completed self-reported questionnaires. Actor–partner interdependence model (APIM) revealed that both grandchildren’s and grandparents’ reported family resilience positively associated with their own SWB but not the other’s SWB. Grandchildren’s and grandparents’ perceived relationship quality mediated the association between their perceived family resilience and their own SWB. Moreover, grandchildren-perceived relationship quality mediated the association between grandchildren-perceived family resilience and grandparents’ SWB. Meanwhile, grandparent-perceived relationship quality mediated the association between grandparent-perceived family resilience and grandchildren’s SWB. The findings highlighted the significance of family resilience and relationship quality in boosting the two generations’ SWB in CLBF. The results also suggested that left-behind grandchildren and their grandparents depend on each other. Future intervention programs could be benefitted from integrating the enhancement of family resilience and improvement of grandparent–grandchild relationship quality to promote the well-being of left-behind family members.