Grandmothers are important in Chinese families. This study explored the early emerging mother-grandmother-infant network and its association with child's socioemotional development in multigenerational families in a non-WEIRD country. The analytic sample included 60 children (T1: M[subscript age] = 6.5 months) and their caregivers residing in Beijing. Measures used were the Strange Situation Procedure (SSP), the Lausanne Trilogue Play (LTP), the Maternal Behavior Q-Sort (MBQS), and the Infant-Toddler Social and Emotional Assessment. Structural equation and path modeling revealed that (1) more grandmaternal neutral/watching coparenting behaviors at the first assessment were related to more secure infant-mother attachment relationships at the second assessment (T2: M[subscript age] = 1 year); (2) maternal sensitivity at T2 was a partial mediator between earlier undermining and neutral/watching coparenting behaviors and young children's externalizing problems at the final assessment (T3: M[subscript age] = 2 years). Findings are discussed in terms of the roles played by mother-grandmother coparenting network in the children's socioemotional development.