The relationship between executive function and second-language ability remains contentious in bilingual children; thus, the current study focused on this issue. In total, 371 Uyghur-Chinese bilingual children ranging from three to six years old were assessed by a battery of tasks measuring language ability (expressive vocabulary tests, receptive vocabulary tests, and phonological awareness of both their native language Uyghur and second language Chinese) and executive function (working memory, inhibition, and switching). Our results indicated that age is a crucial moderator of the relationship between second-language ability and executive function. Specifically, executive function unilaterally predicted second-language ability in children who were 3-4 years old, whereas second-language ability and executive function bilaterally influenced each other in children who were 4-5 or 5-6 years old. These findings suggest that executive function and second-language ability have an intertwined and causal relationship among preschool children during development. The theoretical implications of these findings are discussed. This article is protected by copyright. All rights reserved.