Learning motivation is one of the important factors that affect students' mathematics learning. It is one of the challenges that junior high school mathematics unit teaching faces to effectively improve students' learning enthusiasm in mathematics unit classroom. ARCS motivation model is an important theoretical model that focuses on the relationship between instructional design and students' motivation, and can effectively stimulate and maintain students' learning motivation in subject teaching. Based on this, this study applies ARCS motivation model to junior high school mathematics big unit teaching classroom, and discusses the strategies of motivating junior high school mathematics big unit teaching motivation under ARCS motivation model, that is, effective introduction of teaching, selection of related content, phased task driving and multiple feedback evaluation; The teaching mode of junior high school mathematics big unit under ARCS motivation model is constructed, that is, analyzing the demand and setting the goal; Generate strategies and integrate teaching; Implement strategy and encourage design; Unit evaluation and evaluation results. This research is an in-depth exploration of cultivating students' autonomous learning ability in large-scale unit teaching, and is committed to improving the teaching effect of large-scale units and implementing the new requirements of compulsory education curriculum standards.