The invention provides a precise separator assembling device for an oil-gas separator assembly and belongs to the technical field of assembly. By the adoption of the precise separator assembling device for the oil-gas separator assembly, the problems that in the prior art, a precise separator assembling of an oil-gas separator is difficult, and manual installation efficiency is low are solved. The precise separator assembling device for the oil-gas separator assembly comprises a main machine frame and an auxiliary machine frame. A downward pressing air cylinder is fixed to the top of the auxiliary machine frame. A piston of the downward pressing air cylinder is connected with a cover plate clamp of a precise separator. A metal valve plate positioning groove for the precise separator is formed in the bottom of the auxiliary machine frame and corresponds to the cover plate clamp. Holes corresponding buckle feet of a cover plate are formed in the bottom of the positioning groove. Buckle foot supporting rods which can move up and down along the holes are matched with the holes. The lower ends of the buckle foot supporting rods are connected with the air cylinder. A metal valve plate storing area, a metal valve plate sucker and a moving mechanism of the metal valve plate sucker are arranged near the positioning groove. The precise separator assembling device for the oil-gas separator assembly is simple in structure, low in cost and capable of improving assembling efficiency greatly.