Piston pump is the power source and key component of hydraulic system in construction machinery． Its working status and service performance directly affect the construction quality and operation safety of the construction machinery． Aiming at the harsh working environment and the variable load working condition of the piston pump，different severity levels of loose slipper faults were simulated based on the virtual prototype model of piston pump． The dynamic responses of the piston pump under different loading and structural health conditions were investigated through simulated analysis． And a fault diagnosis method of piston pump in construction machinery under variable load condition was proposed in this paper． According to the method，the axial vibration signal of piston pump is collected at first． Then the vibration signal is divided into several equal parts and the ＲMS of each segmented signal is calculated． At last，the loose slipper failure can be detected based on the gradient of the ＲMS trend line． The experimental results show that the proposed method can effectively detect the loose slipper of piston pump in construction machinery under variable load working condition．