Abstract: The influence of valve settings pretension force on the pressure difference caused by deflected disc at the valve-opening point is studied in this paper. Taking piston valve setting for instance, based on finite element simulation and theoretical calculation, the mathematical relationship between pretension force and the pressure difference caused by deflected disc at the valve-opening point is found. Based on the mathematical model, the influence of piston valve bodys elastic modulus and deflected discs elastic modulus are analyzed. Finally the paper shows that the smaller the elastic modulus of piston valve body, the greater influence the pretension force has on the pressure difference, and the influence of the change of the elastic modulus of deflected disc on pretension forces function is very small.