The present invention discloses an in-mold assembly injection molding method for an injection molded part. The injection molded part is molded by an injection mold. The injection mold comprises an upper plate and a lower plate. A molding cavity for molding the injection molded part is formed between the upper plate and the lower plate. When the injection mold is opened, the injection molded part remains on the lower plate; the injection molded part includes a main body and a cover body connected with the top of one side of the main body through a hinge; a push rod for pushing the main body ismovably provided through the lower plate; and the cover body is arranged horizontally. The injection mold also comprises an assembly mechanism. When the injection mold is opened, the cover body coverson the main body through the assembly mechanism so as to realize the in-mold assembly of the injection molded part. In the in-mold assembly injection molding method for the injection molded part of the invention, the assembly mechanism is added in the injection mold and the injection molded part is assembled by the assembly mechanism after molding, thereby the labor cost is reduced and the production efficiency is improved.