The screw assembly stress and stress relaxation of torque motor affect the pressure output performance of the pressure servo valve pre-stage and zero position. To quantify the influence law， the assembly stress analysis of pressure servo valve’s torque motor is carried out by using ABAQUS， and the influence of the screw assembly stress and assembly stress relaxation at different positions on the four zero air gap values is studied. The mathematical model of pressure servo valve is established， combined with the nonlinear lumped parameter model of pressure servo valve which is built in the MATLAB/Simulink， and four zero air gap values which are influenced by the screw stress relaxation at different positions are used as simulation input. The influence of assembly stress relaxation on the zero position and the output pressure differential of pressure servo valve’s nozzle cavity are analysed. The influence law of the screw stress relaxation in symmetrical and asymmetrical position on the zero position and the output pressure difference of pressure servo valve pre-stage are obtained. This approach provides a theoretical foundation for the screw assembly process improvement and zero position drift failure diagnosis in the pressure servo valve.