Abstract: In order to solve the contradiction between safety damming and downhole filling of tailings dams, the effects of particle size on tailing strength and dam stability are studied. The tests show that, the weighted average particle size of tailings decreases slowly with the increase of dry beach length, the relationship between the weighted average particle size of tailings and the length of the beach is the power function deposition model. The internal friction angle of tailings decreases with the decrease of the weighted average particle size, and the internal friction angle of tailings decreases significantly when the weighted average particle size is less than 0.05 mm. The stability of tailings dams decreases with the decrease of the weighted average particle size, and the stability of dam decreases significantly when the weighted average particle size is less than 0.05 mm. The results of this study provide a reference for the boundary particle size of tailing sand for safe dam construction and coarse material-filled underground.