The analysis of motion and mechanics property has been studied on the five hinged incline arranged and double elbowed force increasing mechanism of injection machine in this paper. An optimization design is proposed on the force increasing mechanism by use of genetic simulated annealing algorithms. A complete procedure of optimal design is introduced so as to increase the stroke ratio and the amplification of the force, and to decrease the total length of mechanism, which belongs to multi-object optimization problem. Compared with the traditional methods, the result shows that the stroke ratio is increased, the amplification of the force is increased and the total length of mechanism is decreased. Moreover a sensitivity analysis of design parameters has been performed to see changes in injection performance parameters, and results show that the length of back elbowed bar and the length of connected bar have a significant impact on the performance measures. And the results recommend that the close clearance of the length of back elbowed bar and the length of connected bar must be maintained.