In the urban-rural dual structure context, social anxiety among left-behind children has become a focus of social attention. Based on the social anxiety mechanism model, the occurrence mechanism of social anxiety in left-behind children was analyzed. The intervention experimental research method was used to intervene experimental subjects. The results showed that there was no significant difference (p>0.05) in the scores on the Social Anxiety Scale for Adolescents (SAS-A) between the experimental group and the control group before intervention. After the intervention, there was a significant difference (p<0.01) in the scores on the Social Anxiety Scale for Adolescents (SASA) between the experimental group and the control group. This indicates that group painting art therapy can significantly reduce the level of social anxiety of left-behind children and has a positive effect on improving their self-focus and external attention. The promotion of group painting art therapy and the joint efforts of the state and society can help alleviate the social anxiety of left-behind children.