The radial force of annular gear and the flow pulsation is analyzed based on internal gear pump with four idlers. The model of the dual objective function is established with the minimum flow pulsation and the minimum volume of internal gear pump with four idlers,and the model is solved by using the genetic algorithm.The results show that the radial force of annular gear is balanced. Comparing with common internal gear pump,the flow pulsation of internal gear pump with four idlers is decreased greatly,and the flow pulsation is decreased with the increase of the number of annular gear teeth and increased with the increase of the number of small gear teeth. The optimization results meet the requirements of reducing the flow pulsation and weight,so that the theoretical basis can be provided for the design and manufacture of internal gear pump with four idlers.