The invention provides a proportional pressure reducing valve for waterworks. A cylindrical valve body is arranged, wherein a water inlet is formed in the front end surface of the valve body and a water outlet is formed in the rear end surface. The diameter of the water outlet is larger than that of the water inlet. An annular convex shrinkage part is formed on the inner wall between the water inlet and the water outlet on the cylindrical valve body in a swelling manner. A piston slidingly and freely penetrates and is inserted in the shrinkage part in an air-tightened manner. A valve part for opening/closing the water inlet is arranged on the end of the piston. A hollow part is arranged on the piston, wherein the base end of the hollow part is opened on the water outlet. A plurality of water flowing openings are formed on periphery of end side, which is more forward from the shrinkage part, of the piston. A flange is formed on the whole periphery of hte base end of the piston and slides on the inner wall, which is more backward than the shrinkage part of the valve body, in an air-tight manner. An air tightness chamber arrnaged between the flange and the shrinkage part is communicated with the hollow chamber. The hollow chamber is formed by air-thightly sealing a concave part formed by sinking peryphery of outer wall of the valve body with a cylindrical cover outside-embedded into the valve body.