Depression is highly prevalent among left-behind children (LBC) in China. Previous research has suggested that attachment relationships may be linked to depression in LBC; however, the connections between different types of attachment relationships and depression, as well as the potential mechanisms involved, remain underexplored. This study utilized a longitudinal design to investigate the associations between multiple attachment relationships and depression among LBC, focusing on the mediating role of self-compassion. A sample of 604 elementary school LBC completed a series of questionnaires assessing mother-child attachment, father-child attachment, peer attachment, teacher-student relationships, self-compassion, and depression at two time points: baseline (T1) and six months later (T2). Results indicated that, among the four types of attachment relationships, only teacher-student relationships negatively predicted depression. Self-compassion mediated the associations between multiple attachment relationships and depression, such that higher levels of multiple attachment relationships enhanced self-compassion, which subsequently reduced depressive symptoms. Furthermore, the predictive effect of the teacher-student relationship on self-compassion was more significant than that of the other three attachment relationships, which did not differ significantly in their prediction of self-compassion. These findings expand existing knowledge regarding the roles of attachment relationships and self-compassion in LBC, highlighting that self-compassion and teacher-student relationships are significant predictors of later depression in this population.