Parent-grandparent coparenting has become increasingly common in Chinese families. The current study elucidated the associations between parenting attitudes, mother-grandmother coparenting relationships, and preschoolers’ social adjustment in 552 mother-grandmother dyads. Utilizing multi-informants’ reports, mothers and grandmothers reported on their respective parenting attitudes, perceptions of their mother-grandmother coparenting relationship, and the preschoolers’ social adjustment (measured by two indicators: child behavioral and emotional problems, child competencies). Results demonstrated that: (a) The mothers’ adaptive parenting attitudes were directly associated with higher child competencies, while the grandmothers’ adaptive parenting attitudes were directly associated with fewer child behavioral and emotional problems; (b) The mothers’ and grandmothers’ adaptive parenting attitudes were indirectly linked to the preschoolers’ social adjustment through their perceptions of the mother-grandmother coparenting relationship; (c) Significant crossover effects were observed, wherein the mothers’ adaptive parenting attitudes were associated with the grandmothers’ positive perception of the mother-grandmother coparenting relationship, and vice versa. Overall, the present study extended the ecological model of coparenting to Chinese parent-grandparent coparenting families, underscoring the necessity to consider the interactive and differential impacts of intergenerational caregivers on preschoolers’ social adjustment.