Manten mobile stage semi trailer is a kind of trailer which is convenient for mobile performance and can be expanded into a stage. It mainly includes two parts: the mechanism semi trailer and the upper mounting part. The upper mounting part consists of the top plate, the side plate and the bottom plate. The side plate and the top plate are respectively expanded by four hydraulic cylinders to form the top of the stage. The bottom plate is expanded by four hydraulic cylinders to form the stage, eliminating the traditional platform building process, saving time and labor, and easy, safe, reliable, functional, and beautiful.