Based on the moving particle semi-implicit (MPS) method and simulation tests, the splash lubrication behavior and oil churning loss mechanism of planetary gear trains were studied, and a numerical calculation model for oil churning loss of planetary gear trains was established. The research results indicate that during the operation of the planetary gear train, the internal space of the gearbox can be divided into oil-pool zone, oil-dragging zone and oil-deficient zone based on the distribution characteristics of the oil inside the gearbox. As the planetary gear passes through the three zones successively, its oil churning loss torque exhibits a periodic variation pattern. Based on this research, the oil immersion coefficient curve of the planetary gear was fitted, and a numerical calculation model for the oil churning loss of the planetary gear train was established. A comparison between the numerical and experimental test results reveals that, under different rotational speeds and immersion depths, the maximum error between them does not exceed 25%.