In the injection molding of large and thin plastics parts, multiple gates are used, so many weld lines would generate because melted plastic has more than two directions of flow. The existence of weld 1ine harms not only appearance, but also the mechanical properties of products. The paper studied the method of sequential injection to solve this problem. Taking car door trim panels for example, the flow simulations were done by Moldflow software. Conventional hot runner injection and sequential hot runner injection, different gating locations were compared. Optimization result was got. It was proven that sequential injection system can improve the quality of the surface (especially the weld 1ine) and the performance of the actual products thereby this can be guiding significance for the practical production.