By analyzing the operating principle and building the mathematical model, the pressure characteristics of the tri-proportion controller are obtained. A simulation about the pressure characteristics is carried out, which shows the differential pressure and rotational speed are subjected to a linear relationship within the full measuring range. The simulation result agrees with experimental data very well, which shows the feasibility of this simulation method. Analyzing the pressure characteristics and the proportional control accuracy, we obtained the minimum differential pressure, the maximum differential pressure, the factors which affect the differential pressure and the relationship between the differential pressure and leakage. And finally we obtained the solutions of decreasing the leakage and improving the proportional control accuracy.