A structural improvement of piston set for use in hydrant is provided; a hydrant main body having a valve chamber for receiving a water control valve is mainly provided, the water control valve is composed of a valve member, rubber pads, a piston cylinder and a piston rod disposed inside the valve member, a valve rod, a cover, a valve cap, and a rotary button; the outer side of the piston cylinder is formed with different outer diameters, and preset portions thereof are formed with a water inlet and a water outlet holes, thus the piston rod can be adjusted to alter the relative relation, thereby achieving functions of starting/terminating water supply and adjusting water mixing ratio; and characterized in that: the piston cylinder is formed through a stainless steel base pipe and a plastic sleeve member enclosing the exterior thereof; accordingly, better industrial application valves are provided.