The invention relates to a small hydraulic self-closing spring self-opening air valve. The air valve is arranged on a water filling air exhaust and water drainage air supply pipeline of a water turbine volute or a water distribution ring pipe and is of a vertical structure as a whole. A sealing ring is installed on the bottom of a piston cylinder, and a shaft sleeve is arranged in the piston cylinder and presses the sealing ring. The external circle of the piston rod is sleeved with a spring, the piston rod and the spring are inserted into an inner hole of the shaft sleeve, the spring is compressed, and one end of the spring makes contact with the shaft sleeve. A plug is fixed to the piston cylinder through threads, the piston cylinder 1 is fixed to a valve body through a screw, and a sealing ring is arranged between the piston cylinder and the valve body. In the processes of water filling air exhaust and water drainage air supply of the water turbine volute or the water distribution ring pipe, due to the fact that water pressure difference, air pressure difference and spring force exist on the two sides of the piston rod, the piston rod moves vertically in the shaft sleeve to control the through-flow area of the piston cylinder and a circular hole in the external circle of the shaft sleeve, and then the effects of air exhaust, water sealing and air supply are achieved.