Based on the analysis of dynamics method of pneumatic transmission, the mathematic model of air brake Load Sensing Proportional Valve (LSPV) is established by fluid servo control theory. The air brake LSPV of one All Terrain Vehicle (ATV) is simulated with the platform of advanced modeling ervironment for performing simulations of engineering system(AMESim), then the input and output pressure characteristic curve of the valves are obtained. The characteristic curve shows, with the load increasing, the pressure adjustment point of Load Sensing Proportional Valve (LSPV) moves up. When the pressure reaches into the pressure adjustment point, the increase rate of output pressure is less than the increase rate of input pressure, so that it can effectively prevent the lockup of the rear wheel precede the front wheel. The result of the simulation is consistent with the experiment result. The validity of the model is verified, which provide foundation for the simulation of the air brake elements and brake system.