In order to better grasp the changes in the mixing uniformity of reclaimed asphalt pavement (RAP), the dynamic process of the evolution of RAP mixing uniformity at different mixing speeds was simulated. Firstly, based on the discrete element method (DEM) to simulate the RAP mixing process under different mixing speeds, we analyze the mixing uniformity change and internal mechanism of the old aggregate agglomeration crushing; Secondly, on the basis of agglomerate crushing, we further analyze the uniformity evolution process of the new and old aggregates; Finally, we use the 3D printing technology and image processing to analyze the mixing uniformity change of the new and old aggregates in the chamber under different mixing speeds. There is a process of agglomerate crushing from fast to slow, The inflection times of the fitted curves of dispersion factors of old aggregate agglomeration at 5, 10, 15, 20 r/min mixing speeds are 10.92, 6.69, 5.48, 4.12 s, respectively. the further the agglomerates are away from the center of mixing, the more obvious the trend of speed and force changes in mixing, and the more homogeneous the mixing is. The inflection times of the fitted curves of disper-sion factors of old and new aggregates at 5, 10, 15, 20 r/min mixing speeds are 17.05, 8.74, 5.66, 4.26 s, respectively. The uniformity of the mixing of the old and new aggregates under different mixing speeds is slightly different between the indoor experiment and DEM, but all of them reach the inflection point of uniformity around 4 s. The change of mixing uniformity of RAP is a complex dynamic process, and the mixing speed is a key factor that cannot be ignored to affect the mixing uniformity.