Pressure relief valve for water system is a direct acting type film spring pressure relief valve. Pressure relief valve is composed of spring, film, piston, valve seat, disc, etc. Pressure relief valve film senses downstream pressure directly to drive disc to control open degree of disc to reduce or sustain pressure. Pressure relief valve suitable for water, air, non corrosive liquid of working temperature of 0-80℃. In high rise building cold-hot water supply system and fire fighting water supply system, pressure relief valve can replace regular partition pipe, to simplify and save device, low project cost.