The invention belongs to the technical field of pumps and particularly relates to a piston pump for a gasoline engine. The piston pump comprises a clutch connected with the gasoline engine, the clutch is further connected with a gear speed changer, an output shaft of the gear speed changer is an eccentric shaft, the eccentric shaft is connected with a plunger through a connecting rod, one end of the plunger extends into a pump body, the front end of the plunger is sequentially provided with a water outlet valve assembly and a piston assembly, the piston assembly is located in a piston valve sleeve and provided with a limber hole, the front end of the piston valve sleeve is provided with a water inlet valve sleeve, a water inlet valve is arranged in the water inlet valve sleeve, and the front end of the water inlet valve sleeve is provided with a water inlet chamber. The gasoline engine is combined with the clutch, and then the pump body and a box body of a gear reducer are integrated, so that the light weight property of the single cylinder piston pump is achieved; the total weight of an existing single cylinder piston pump is only 2.17 KG.