Abstract: Compliant constant force mechanism has been widely used in ultra-precision polishing, micro-operation and other fields because of its characteristics of outputting constant force within a certain range. To improve the constant force range of the constant force mechanism, this paper optimizes the design of the constant force mechanism based on the mechanism theoretical model and genetic algorithm. Firstly, the theoretical model of the positive and negative stiffness mechanism is established, and the constant force characteristics and initial design parameters are determined. Then, the sensitivity analysis of the initial design parameters is carried out to screen the key design variables, the theoretical model of the key variables is analyzed to determine the optimization objectives, and the genetic algorithm is used to optimize the model. Through the finite element method, it is verified that the optimized constant force mechanism in this paper can output a larger constant force range. The simulation results show that the optimized constant force range is increased by 30 % compared with the initial structure. This method provides an effective way for the optimization and application of compliant mechanisms.