Y13H type internal thread pilot operated piston type steam pressure reducing valve belongs to the pilot operated piston type pressure reducing valve. It is composed of two parts: main valve and pilot valve. The main valve is mainly composed of valve seat, main valve disc, piston, cylinder liner, spring and other parts. The pilot valve is mainly composed of valve seat, valve disc, diaphragm, spring, adjusting spring and other parts. Set the outlet pressure by adjusting the spring pressure, use the diaphragm to sense the outlet pressure change, and adjust the size of the overflow area of the main valve throttle part by driving the piston through the opening and closing of the pilot valve, so as to realize the function of pressure reduction and stabilization.