The invention discloses an agricultural automatic water supply device, comprising a water tower. The device is characterized by comprising a transformer, an AC contactor, a DC relay, a first capacitor, a second capacitor, a third capacitor, a first resistor, a second resistor, a third resistor, a first diode, a second diode, a third diode, a fourth diode, a fifth diode, a sixth diode, a seventh diode, a first triode, a second triode, a third triode, a first switch, and a second switch. The device is good in energy conservation effect, compact in structure, small in land occupation, stable and reliable in operation, long in service life, flexible in scheme design, and adjustable in water supply pressure, and flow can be large or small. Intelligent reconstruction makes use of a conventional water tower more convenient, and secondary pollution of water quality can be prevented thoroughly.