BackgroundCallous-unemotional traits and emotional lability/negativity of young children have been regarded as the markers of externalizing problem behaviors. Based on the sensitivity to threat and affiliative reward model and the general aggression model, emotional lability/negativity may act as a mediator in the relationship between callous-unemotional traits and externalizing problem behaviors. Additionally, a positive teacher-child relationship could act as a buffer given the parental absence in left-behind children. However, these links remain unexplored in left-behind preschool children. Therefore, this study explored the link between callous-unemotional traits of left-behind preschool children and externalizing problem behaviors, as well as the mediating role of emotional lability/negativity and the moderating role of a positive teacher-child relationship.MethodData were collected on 525 left-behind children aged 3 to 6 years from rural kindergartens in China. Preschool teachers reported all data through an online survey platform. Moderated mediation analysis was performed to examine whether the mediated relation between callous-unemotional traits and externalizing problem behaviors was moderated by a positive teacher-child relationship.ResultsThe results showed callous-unemotional traits significantly predicted externalizing problem behaviors and lability/negativity acted as a mediator, while a positive teacher-child relationship acted as a protective factor in moderating the relationship between callous-unemotional traits and emotional lability/negativity. This study identified a moderated mediation effect among the four variables in left-behind preschool children in China.ConclusionThe findings provide support for the advancement of theoretical foundations, and provide an avenue for further exploration to support the mental health and overall development of left-behind children during early childhood.