The main purpose of this paper is to design a high pressure regulator which reduces an input pressure varying between 400 to 500 bars to 350 bars constant pressure. First of all, for mathematical simulation of regulator’s performance, the related equations were derived. Because of nonlinearity and coupling, these equations were solved by using numerical methods. Also, the effects of different parameters variation on regulator performance were investigated. One of the most important parameters in regulator performance is the preload of control spring. Deriving the maximum and minimum values of the spring force from the mathematical simulation, the proper spring was designed. In the next step, Finite element analysis, considering thermal effects, was performed using commercial software ABAQUS. To achieve sufficient safety factor, thicknesses were specified by trial and error. The results were in good agreement with the analytical solution.