The invention discloses a prefabricated prestressed composite box girder, including a plurality of box beam units. Each box beam unit includes a bottom plate, a web plate, an end plate and a top plate; the bottom plate is a concrete prefabricated plate whose upper surface is embedded with the multiple of mounting bases in the long direction of the plate; the tops of web plates are provided with connection stiffeners, the number of the web plates is matched with the number of the mounting bases, and all web plates are arranged side by side in the length direction of the plates and detachably connected with the mounting bases; the end plate is placed on the bottom plate and detachably connected between two adjacent web plates; and the top plate is a concrete precast plate, on which a post-pouring hole through the thickness of the plate is reserved, and the top plate is placed on the web plates, so that the post-pouring hole is arranged outside the connection stiffener in a sleeved mode.As all parts are prefabricated in a factory, the construction speed and quality are improved, by adopting the detachable connected web plates, the problem that the main tensile stress of concrete neara bearing is not enough is solved, the additional stress of steel corrugated web plates caused by shrinkage deformation in the curing process of traditional mixed beam concrete plates can be avoided,and the bearing property is ensured.