The domestic 25-ton truck crane has the problems of high system pressure during steady slewing and great hydraulic impact when slewing motion starts and brakes. So the simulation model of the slewing hydraulic system and the core hydraulic components was constructed by AMESim. The flow areas of throttling grooves on the valve rod of the directional control valve were calculated by Matlab and converted into text files, which were imported into the simulation model. To get the load parameters of slewing system in different work conditions, an experiment on real truck crane was performed. Then the problem work conditions recurred in the simulation, which showed the veracity of the simulation model. Based on the simulation model an improving method was presented and simulated and the simulation results showed that through the improving method, the system pressure and the hydraulic impact could be efficiently reduced.