The influence of China's “one-child” policy on the mental health of children and adolescents is still unclear. This study examined the associations between number of children and children's mental health using data from the national school children and adolescents survey of China. Mental health was measured using the Achenbach's Child Behavior Checklist (CBCL). Linear regression models were used to explore the relationship between number of children per family and children's mental health. A total of 64,017 students were enrolled, with 68.60 % from one-child families. Compared with children from one-child families, those from multiple-children families had significantly higher scores on the total CBLC and each of its subscales (all P<0.001). Analyses suggested children from multiple-child families have a higher risk of mental health problems (total problems on the CBCL: adjusted B=2.217, 95 %CI=1.883–2.550, P<0.001). Regular mental health assessments should be implemented and effective interventions should be developed for those in need.