Objective: To examine the relationship between mother’s psychological flexibility and preschool children's anxiety and whether an only child plays a moderating role in it. Methods:387 mothers of preschool children aged 3-7 years were recruited,the only child accounted for 47.8%. A survey was conducted using the second version of the Acceptance and Action Scale (AAQ-II) and the Spence Child Anxiety Scale (SCAS). Results:The results of the group regression showed that AAQ-II scores were negatively correlated with total SCAS scores and scores on 4 dimensions scores of separation anxiety, fear of somatic harm, social fear, and generalized anxiety in preschool only child (β=-0.27, -0.25, -0.18, -0.22, -0.34; all P<0.05). It also were negatively correlated with total SCAS scores and 5 dimensions scores of separation anxiety, fear of physical harm, social fear, obsessive-compulsive-impulsive disorder, and generalized anxiety in preschool non-only children (β=-0.45, -0.37, -0.37, -0.38, -0.33, -0.39; all P<0.001). The results of the moderated effects test showed that the regression coefficients of AAQ-II scores and total SCAS scores in the only child and non-only child groups had statistical significance (P<0.05),that shows whether the child is an only child moderates between the mother’s psychological flexibility and preschooler’s anxiety.. Conclusions: The lower the mother's psychological flexibility, the higher the preschooler's anxiety level, and preschool non-only children are more likely to be affected by the mother’s psychological flexibility.