The utility model discloses an automatic valve for a water intake of a water tank. The utility model comprises an automatic device and a valve, wherein, the automatic device is mainly composed of a mainspring, a mainspring wheel, a transmission mechanism, a control shifting fork, a pressure gauge of return water, an upper clamp plate, a lower clamp plat, a connecting part, a spring, a pipe clamp, a casing, etc. Whether the mainspring wheel of the automatic device is rotated is controlled through a shifting fork, a ratchet wheel and a transmission wheel by using stored return water as a pressure power source; the utility model can be widely used for water intake of a solar water heater tank through the combination of a mainspring wheel and an ordinary switch valve of 90 DEG of a knob. When the water level reaches a limited standard value, the return water flows into the pressure gauge, and the button-control knob automatically rotates back to close the valve; when the valve is opened again, the shifting fork is in an off position, and the automatic device is in the ready state. The use method is the same as the use method of an ordinary on-off valve of 90 DEG of a knob, and just the knob is counterclockwise rotated in 90 DEG. The utility model has the characteristics of safety, practicality, convenient operation, low cost, strong durability, etc.