A numerical simulation is applied to investigate the avalanche flow characteristic of partides in the slumping regime within a rotating drum. Based on the simulation model, the quantitative change of particles involved in avalanches on the free surface is investigated. The growth rate of avalanching particle number experiences a process of increasing, then stabilizing, and finally reducing. Investigation of the nucleation and propagation process of the avalanching reveal that any position of the free surface nucleates uniformly and the values of propagating velocities toward upward, downward. leftward, and rightward directions are constant. Then, it is discovered that the number of particles participating in the avalanches increases linearly with the square of time at the beginning of the avalanche, which is also proved by the simulation result. Finally, we find that the distribution of the rolling angles approximately follows the normal distribution with the same characteristics at different times. Crown Copyright (C) 2020 Published by Elsevier B.V. All rights reserved.