Social anxiety is a problem that needs attention in the social development of rural left-behind children. Analysis of the survey data of 470 rural children in a mountainous county in Zhejiang Province shows that:left-behind children's interpersonal relationships are good,but social anxiety is more serious due to interpersonal cognition;left-behind children's interpersonal relationships significantly and negatively predicted social anxiety;the marginal effect of interpersonal relationship on social anxiety of left-behind children with high family cultural capital was higher;the marginal effect of interpersonal cognition on social anxiety of girls,children with low family cultural capital and leave behind for more than three years was higher. Interpersonal cognition produced a "masking effect" in the process of the influence of interpersonal relationship on social anxiety,and the path of interpersonal cognition on social anxiety among left-behind children was greater than that of non-left-behind children. Good interpersonal relationship can effectively reduce left-behind children's social anxiety,and paying too much attention to other's feelings and thoughts will exacerbate left-behind children's social anxiety. In this regard,it is suggested that parents should strengthen accompanying and caring to build good parent-child relationship;schools should enhance emotional support to create a friendly atmosphere of interaction;and the society should provide cultural facilities to enrich the amateur cultural life,so as to provide effective support for left-behind children to alleviate their social anxiety.