The present invention relates to a construction improvement of the piston valve in compressing pump with features as below: The discharge base, which is built in the center of the piston valve, is contrived into planar shape; Some discharge spouts are created on said discharge base in corresponding with each said inlet slots; An orientating lump is created in the top center of said discharge base and being punched an orientating hole at its center; The anti-backflow plastic gasket, which is contrived into 3-blade planar shape to entirely cover the top of said discharge base, has a gap cleft created between each blade so that each said blade can closely block exactly each said corresponding inlet slots on said discharge base respectively; During each said wobble wheels of the compressing pump pushing said diaphragm in turns, the water flow, which coming from each said inlet slot at each area, will continuously runs into each said discharge spout at each area in said discharge base by turns so that each blade on said anti-backflow plastic gasket working up-open and down-shut action by turns as well, thus it really achieves the open-and-shut effect of each said discharge spout at each area; Therefore, not only the drawback of leakage and pressure loss or failure can be prevented, but also the serving life of said anti-backflow plastic gasket can be prolonged, thus the compressing and discharging efficiency of integral compressing pump can be further enhanced.