Abstract: To explore the movement mechanism of the composite cover layer composed of tailings par⁃ ticle layers in the open-pit to underground transition area and the rock cover layer during its downward movement，based on the structural parameters of the underground mining area in the templegou open-pit�to-underground mine and the composition structure of the composite cover layer，using the same reference model and iterative calculation method of PFC2D numerical simulation experiment，a full-height mining test scheme was designed. The test results show that the deceleration effect of the surrounding rock on the composite cover layer is related to the particle depth and the distance between the particles and the sur⁃ rounding rock. The bigger the particle depth and the greater the distance between the particle and the sur⁃ rounding rock，the weaker the deceleration effect. The influence area of the mining ellipsoid on the com⁃ posite cover layer is approximately an ellipsoid，with the long axis being the maximum acceleration area of 20~40 m. The acceleration effect decreases rapidly in the vertical path direction and towards both sides. During the mining process，the retention amount of the ore particles first decreases and then increases， and it is not a stable increase. The large retention of the ore mainly occurs in the deceleration area near the surrounding rock after the fifth section of the mining. The difference between the amount of cover layer re⁃ lease and the retention amount of the ore is small. The additional thickness required for the cover layer to meet the demand is approximately 10 m. The release of the ore has a significant effect on the movement of the underlying rock in the composite cover layer，causing the overall deformation of the modified tailings particles in the upper part to be relatively large. The thickness of the cover layer changes slightly，but due to the depression at the upper edge of the composite cover layer，from the perspective of preventing water disasters，it is recommended to supplement the modified tailings particles to the middle area. The research conclusion provides data support and theoretical basis for the technical realization of using modified tail⁃ ings particles as the cover layer.

Key words: composite cover layer, numerical simulation, migration mechanism