The main function of accumulator acts as an emergency power supply in megawatt fan drift hydraulic system. Research the influence of accumulator on the yaw and braking performance of hydraulic system through simulation, and calculating the reasonable value range of its precharge pressure and prefilled volume have great significance. The simulation model of accumulator served as an emergency power supply in AMESim was build up. The influence law of prefilled volume and precharge pressure on yaw performance and braking performance of system were researched. Results show that accumulator acting as an emergency power supply can meet the pressure demands of the system during the working process of yaw and brake, and within the set range the greater the precharge pressure and prefilled volume is taken, the more reliable the working performance of the system is.