Diamond has attracted extensive attention from many scholars because of its unique properties. However, there are still bottlenecks in how to achieve high efficiency polishing and global flattening of diamond which restrict its application. The aim of this study is to obtain a globally homogeneous flattened diamond surface in macro dimension by introducing a compound motion of the polished workpiece. It is shown that ultra-smooth diamond surface machining with global flattening can be realized by UV-assisted low-speed dynamic friction polishing technique, and the typical roughness is 0.175 nm as measured by 3D optical surface profiler. These experimental results demonstrate that the catalytic phase transition process of UV light is the underlying mechanism to realize the diamond surface polishing under low rotational speed conditions. Additionally, the composite motion of the diamond sample with the metal disk brings large enhancement to the effect of polishing global flattening on the sample surface. The theoretical and experimental studies in this paper provide novel ideas for achieving efficient and polished global flattening of diamond. Display omitted •Our team previously proposed a diamond polishing technique, which was optimized and improved according to 3DM-DFP in this paper.•Ultrasmooth diamond surfaces with surface roughness Rq=0.218 nm and Ra=0.173 nm were obtained.•The comprehensive polishing mechanism is discussed, and the theory of diamond diffusion removal is expanded.•The kinematic analysis of the composite motion process was carried out, and the optimal motion parameters were theoretically obtained.