There is redundancy in the design of suspension mechanism of electric farm vehicle. In order to optimize the suspension arm mechanism, a rigid-flexible coupling model of suspension mechanism is established. Through multi-body dynamics simulation, the maximum force of each hinge point of the suspension arm mechanism in the coordinate axis direction is determined. Through the single variable static simulation test, the single variable numerical fitting model between the maximum equivalent stress and the four structural parameters of the upper arm mechanism has been established. Through the orthogonal simulation test, the multivariable numerical fitting model between the maximum equivalent stress and the four structural parameters is established, and the simulation value and the calculated value of the numerical model are compared to verify the reliability of the model. Using the same method, a multi-variable numerical fitting model between the mass of the suspension arm and the four structural parameters is conducted. Finally, the optimal design parameters are obtained by genetic algorithm with the objective of minimizing the maximum equivalent stress and mass. The optimization results show that the optimized upper arm mechanism has relatively lower mass and greater strength, and its structure is more reasonable. This method could provide reference for other design.