<p>Extensive evidence has consistently supported the association between low family socioeconomic status (SES) and adolescents’ poor academic achievement; however, the underlying mechanisms, particularly within multi-child households, remain less understood. This study aimed to investigate how family dynamics impact academic achievement among adolescents from varying SES backgrounds through exploring the mediating role of marital conflict and marital intimacy, as well as the moderating role of sibling warmth in the longitudinal relationship between family SES and adolescent academic achievement. A total of 473 Chinese adolescents, aged from 10 to 16 years (Mage = 12.37 ± 1.15 years; 57.5% females), participated in a two–wave longitudinal study with a one-year interval. The study used longitudinal moderated mediation analysis via the PROCESS macro. The results revealed that SES directly influences academic achievement and that marital conflict (not intimacy) can serve as a mediator. In high-SES families, marital conflict is less common, and adolescents tend to have greater academic achievement. Conversely, in low-SES families, marital conflict is more prevalent, and adolescents often achieve lower academic performance. Unexpectedly, high levels of sibling warmth exacerbated the negative impact of marital conflict on academic achievement, but the moderating effects on the path from SES to marital conflict were not significant. These findings expand research on the adverse effects of low socioeconomic status on academic achievement in Chinese multi-child families. It also emphasizes the importance of family relationships in promoting academic success and suggests the potential for effective interventions across the entire family system.</p>