work investigates the efficiency of the chemical mechanical slurry (CMS) polishing process by comparing surface reaction modes by ionic and solid reaction modes when polishing the yttrium aluminium garnet (YAG) crystal. The study procedures aim to clarify the polishing performance corresponding to the two reaction types. Experiment results show that the balance between mechanical and chemical effects when using CMS polishing technology can be achieved with the ionic reaction mode. The surface quality when polished by CMS technology with ionic surface reaction modes is better than that with solid surface reaction. The results also show that the ionic surface reaction modes provide more uniform material removal than the solid reaction on YAG crystal surfaces. The resulting polishing when balancing mechanical and chemical action creates an ultrasmooth, without scratch YAG crystal surface, with a roughness obtained in the nanometre form (Ra = 1.031 nm).