We investigate the flow patterns of irregular sand particles under avalanching mode in a rotating drum by using the spatial filtering velocimetry technique. By exploring the variations of velocity distribution of granular flow, we find a type of avalanching pattern of irregular sand particles which is similar to that of spherical particles flow. Due to the fact that the initial position of avalanche in this pattern locates at the middle of the drum and the avalanche propagates toward the edge area gradually, we named it as mid-to-edge avalanching pattern. Furthermore, we find another avalanching pattern which slumps from the edge and propagates toward the opposite edge of the flow surface, named as edge-to-edge pattern. By analyzing the temporal and spatial characteristics of these two types of avalanching patterns, we discover that these two types of avalanche patterns are caused by that the avalanching particles constantly perturb the axial adjacent particles. Thus, the particles on the flow surface are involved in avalanching sequentially in order of the axial distance from the initial position.