The invention relates to a preparation method of an environment-friendly metal processing base oil. The base oil is composed of common cut III distillate oil, vacuum cut I distillate oil, vacuum cut II distillate oil, and vacuum cut III distillate oil of naphthenic thickened oil; the raw materials are subjected to a common hydrogenation treatment or carry out reactions in the presence of a hydrogenation refining catalyst; and an one-step hydrogenation technology or a two-step hydrogenation combined technology is used to prepare the environment-friendly metal processing base oil. The provided base oil has a property viscosity, enables the oil product to have good cooling property, cleanness, rapid penetrating performance, and good surface adsorption performance. The base oil has a reasonable chemical formula and composition, thus has the advantages of good low temperature fluidity, emulsifying property, extreme pressure property, antioxidant performance, and low content of polycyclic aromatic hydrocarbon, and can guarantee that an oil product has a good antioxidant property, safety, and low (no) toxicity.