Mathematically poor students exhibit poor academic performance, as well as increased mental illnesses and behavioural issues; however, the learning dilemma faced by mathematically poor students and the factors affecting their classroom engagements remain unclear. To address this issue, the Perceived Maths Teacher Support Scale, Classroom Engagement Questionnaire, Maths Anxiety Scale and Academic Self-Efficacy Scale were administered to 486 junior school students in Henan Province, China. After determining the differences between the mathematically poor students and a matched control group, a moderated mediation model was constructed to examine the internal influence mechanisms of the mathematically poor students' classroom engagement. The results demonstrated that, compared with the control group, the mathematically poor students exhibited higher maths anxiety and lower classroom engagement, academic self-efficacy and maths teacher support. Moreover, the interaction between the environmental (maths teacher support) and personal factors (academic self-efficacy and maths anxiety) affected the mathematically poor students' classroom engagement. These results enrich the research on mathematically poor students and further elucidate the effective promotion strategies for improving their classroom engagement.