Transmission hydraulic system is very important of influence for the transmission performance. The hydraulic control system of a loader transmission was analyzed, including the main regulator system and shifting clutch. AMESim hydraulic simulation software was used to establish the control model of main regulator system, shifting clutch and the overall hydraulic system. The simulation result shows that the system is able to meet the pressure requirements,and aimed to the hydraulic system of transmission, the influence of timing of engagement and disengagement on shifting clutch was analyzed, thereby the optimal timing was obtained.