Aiming at the thermal load problem of the four-stroke high-speed Diesel engine piston, a piston thermal fluid-solid coupling model based on the combustion thermal boundary and the two-phase flow oscillation cooling thermal boundary is established. The model considers the problem that the piston cannot fill the cooling cavity due to the reciprocating motion. The effects of different engine speeds and the injection speed on the filling rate are studied. The variation curves of the filling rate of the oil in the cooling cavity are simulated, and the transient heat transfer coefficient and temperature of each crank angle are obtained. The average value is then analyzed by thermal flow-solid coupling method, and the influence of the filling rate of the piston cavity on the temperature field of the piston is obtained. Through the comparison of the experimental results of the hardness plug measurement method, the calculation of the model is accurate and can be well used for the simulation of the piston temperature field and the evaluation of the thermal load at the critical position. Based on this model, the regularity analysis of the influencing factors of the piston thermal load is carried out. The influencing factors include the filling rate of the cavity, the air-fuel ratio, the injection timing, etc., and finally the engine operating range that meets the heat load requirements is obtained.