Chinese left-behind families (CLBF) refers to families in which both parents are rural-to-urban migrant workers while their children remain in rural regions under the care of grandparents. CLBF has become an issue of growing public concern. However, the role of relationship quality and personal resilience in promoting grandparents’ and children’s well-being in these families has not been fully recognized. Informed by the family systems theory and the ecological view of well-being, this study aimed to investigate how grandparent–child dyads show reciprocal associations between relationship quality and subjective well-being (SWB) and the extent to which resilience accounts for such reciprocal associations. Self-reported data were collected from 120 left-behind grandparent–child pairs. Analyses using the actor–partner interdependence model indicated that both grandparents and children perceived level of relationship quality showed significant associations with SWB of their own (i.e., actor effects) and SWB of the other (i.e., partner effects). Children’s resilience mediated all of the actor and partner effects of relationship quality on SWB, whereas grandparents’ resilience only mediated their actor effect and children’s partner effect. These findings indicated the interdependence of grandparents and children, and highlighted the importance of grandparent–child relationship quality and resilience in promoting SWB. Future intervention programs are suggested to integrate the enhancement of grandparent–child relationship quality and the building of resilience to boost the well-being of left-behind family members.