Abstract: An pump casing for a sheet metal pump includes a pump body made by pressing or roll forming a sheet metal, a reinforced member which includes a water guide unit and a flange unit separately made by casting or CNC machining and a seal ring engaged with the water guide unit. The pump body is a simple basin shape which can be produced with less stress concentration. The water guide unit may be made with a more precise volute for achieving better pumping efficiency. The reinforced member may increase structural strength of the pump casing for withstanding greater pumping pressure and external forces. The seal ring may be positioned precisely without dislocation because it engages with the water guide unit rather than the pump casing. The gap between the seal ring and the impeller may be kept small and constant, thus obtaining better pumping efficiency, lower operation noise and greater durability.