Diamond has attracted extensive attention from many scholars due to its characteristics, whereas the high-efficiency and ultra-low damage machining regarding diamond is still a bottleneck restricting its applications. Herein, a new polishing equipment was built, and a new method of photochemical mechanical polishing (PCMP) combining mechanical + chemical + optical field coupling was proposed to solve the problem of high-efficiency and ultra-low damage machining of diamond. The experimental results show that the sub-nanoscale surface of Ra 0.071 nm, rms 0.090 nm, Rz 0.943 nm, and the micro removal rate per hour can be obtained after PCMP for the first time. Transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) results manifest that the damage of the diamond substrate after PCMP is amorphous carbon damage, and the damage thickness of amorphous carbon is only 0.6 nm. The PCMP model of ReaxFF molecular dynamics (MD) simulation was constructed to elucidate the microscopic mechanism during the diamond PCMP process. Simulation results show that ultraviolet (UV) irradiation increases the activity of diamond by decreasing the stability of substrate and weakening the C-C bonds, thus promoting the oxidation of diamond surface. Our experimental and theoretical research provides an insight for realizing high-efficiency and ultra-low damage machining of diamond.