Abstract The clamping mechanism plays an important role in obtaining high-quality products of the injection molding process. The clamping mechanism of five-point double-toggle has been widely used for the high-speed plastics injection molding machine. The purpose of this paper is to optimize the five-point double-toggle clamping mechanism through multi-body dynamics analysis. This work also provides guidelines and a clear understanding for designing the clamping system in an injection molding machine with various clamping forces. The theoretical calculation has been handled first and then the computational model has been verified in this study. In addition, the effects of clamping forces on the main dimensions, including movable-fixed plate thickness, tie-bar diameter and average link cross-section have been investigated theoretically and numerically. The results show that the optimal design allows reaching a high force amplified ratio and that the obtained mechanism has good kinematic performance and works steadily with lower energy consumption and lower cost than the preliminary design. Moreover, the relationships between the parameters such as the critical angles of the double-toggle clamping mechanism, the ratio of force amplification and the stroke of movable mold have been found in this work. The optimized parameters will yield useful knowledge to design and manufacture the clamping mechanism of the micro injection molding machine in practice.