This study aimed at exploring the formation mechanism of mathematics self-concept of left-behind children and the influence mechanism of mathematics self-concept on their mathematics achievement. In study 1, the formation mechanism of mathematics self-concept was examined using multilevel analyses of 874 left-behind children. The results showed that (1) the individual mathematics achievement of left-behind children had a significant positive predictive effect on mathematics self-concept, Chinese achievement had a significant negative predictive effect on mathematics self-concept, and the class-average mathematics achievement had no significant effect on mathematics self-concept; and (2) when left-behind children were the majority group, there was an assimilation effect on downward interpersonal social comparison. When left-behind children were the minority group, there was a contrast effect on upward intergroup social comparison. In study 2, the influence mechanism of mathematics self-concept of 585 left-behind children was investigated with a tracking research. The results suggested that (1) Time 1 mathematics self-concept had significant effects on Time 2 mathematics school engagement at between-group and within-group levels; and (2) Time 2 mathematics school engagement played a partial mediating role between Time 1 mathematics self-concept and Time 2 mathematics achievement at the within-group level. These results indicate that the formation and influence mechanism of mathematics self-concept of left-behind children is influenced by contextualized characteristics.