Visual working memory (WM) plays a pivotal role in integrating fragments into meaningful units, but no study has addressed how visual WM integration takes place in children. The current study examined whether WM integration emerges once preschoolers master Gestalt cue and can retain two representations in WM (automatic integration hypothesis), or still needs time to mature (maturation-of-integration hypothesis). Four experiments (N = 168, 81 females, 4- to 6-year-olds, Chinese, in Hangzhou, China, from 2016.10 to 2021.11) were conducted. Although 4-year-olds can retain two objects in WM and benefit from Gestalt cues in simultaneous display (Cohen's "ds" >1.00), they failed when memory arrays were presented sequentially. Meanwhile, 5- and 6-year-olds consistently demonstrated WM integration ability (all Cohen's "ds" >0.69), supporting the maturation-of-integration hypothesis.