Based on the research of principle error，from the angle of independence, for different impact factors of constant power control function, the mappings from the swash plate angle to work pressure and output flow are established. It was calculated of the function of swash plate angle to error and function of impact factor to error on the maximal displacement of swash plate under the action of different influence factors, and analyzed of the influence degree of each influence factor on the original reason of error and error change trend. Provide theoretical basis for key structure design of variable pump with constant power.