AbstractBackgroundMathematics homework is highly prevalent in East Asia. Teachers and parents expect mathematics homework to improve students' performance; however, studies have not clearly defined the effectiveness of the assignment of different amounts of homework.AimsThis study analyses the differential effect of homework amount on various facets of Chinese students' mathematics achievement using a longitudinal design. It aims, thereby, to contribute to the existing knowledge of homework effectiveness and mathematics learning and to provide insights for student development.SampleThe sample included 2383 grade 9 students from a mid‐sized city with a moderate level of economic and educational development in central China.MethodsWe administered a student‐reported questionnaire, collected school reports of mathematics achievements to track retrospective longitudinal variations in mathematics over half a year and analysed the differential effectiveness of homework on mathematics achievement at four hierarchical cognitive levels with Hierarchical Linear Model.ResultsThe results indicated that assigning more homework at the class level could enhance students' mathematics achievement at a low cognitive level, although this effectiveness tended to disappear when the set homework time reached about 1.5–2 h. We did not find evidence that assigning more homework enhances students' mathematics achievement at higher cognitive levels.ConclusionsOverall, this study provides evidence on the effectiveness of homework and offers implications for educational practice and future research.