Abstract Increasing students’ academic achievements is one of important goals for educators, which has received considerable attention in public. To achieve it, it is critical to find out the factors influencing academic success and how they make a difference, one of which is home-school cooperation. In recent years, home-school cooperation, largely defined as “parents’ positive interaction with schools with purpose of facilitating their children’s educational outcomes and future success”, has drawn more attention. Studies have revealed that home-school cooperation was an important factor for students’ academic achievement; however, less is known about the underlying processes that may mediate or moderate that relationship. To answer this question, a moderated mediation model was proposed. Therefore, present study aimed to examine the association between home-school cooperation and academic achievement, as well as the mediating effect of parent-child interaction and the moderating effect of only child situation between them. The survey method was conducted with the home-school cooperation questionnaire, parent-child interaction questionnaire as well as academic test, being administered to 2380 adolescents and their parents. Data was collected and analyzed with Mplus7.0 and SPSS19.0. Confirmatory factor analyses indicated that both model fit the data satisfactorily for home-school cooperation and parent-child communication, and bias-corrected percentile Bootstrap method was used to analyze the mediating effect of parent-child interaction and the moderating effect of only child situation between home-school cooperation and academic achievement. The main findings of the study were as follows: (1) Home-school cooperation had a positive effect on academic achievement among adolescents directly; (2) Parent-child interaction played a mediating role in the relationship between home-school cooperation and academic achievement; (3) Only child situation moderated the relationship between home-school cooperation and academic achievement (for the second half mediation path). In other words, moderated effect was stronger for only children than non-only children. Thus, both mediating and moderating effects existed in the association between home-school cooperation and adolescents’ academic achievement. To conclude, this study constructed a moderated mediation model of parent-child interaction and only child situation to reveal the mechanisms underlying the relationship between home-school cooperation and academic achievement, which could help us comprehend how and when home-school cooperation impacted adolescents’ academic achievement. Furthermore, the results also suggested that parents can take a series of measures to improve the academic performance of children. First, parents can communicate more with schools, increasing the frequency and quality of home-school cooperation. Second, parents could take the initiative to establish a harmonious parent-child relationship with their adolescents. Moreover, parents need to pay more attention to each child as much as possible. Also, the conclusions drawn from this study should be considered in light of some limitations. First, there may be other mechanisms underlying the associations. The second limitation is that, as a cross-sectional study, it’s unable to explore the impact of home-school cooperation on adolescents’ academic achievement at different stages. The future research could improve it by focusing on longitudinal studies.