A water-saving and anti-freezing system of a solar water heater comprises a water tank, water inlet and outlet pipelines, an inverted U-shaped pipe, an air pump, an auxiliary sealing box, a valve 1 and a valve 2. When the water is added to the water tank, a flow sensor detects the water flow, the air pump stops. After the addition of water is finished, a flow sensor sends signals, a controller times X seconds when receiving the signals, the air pump begins to work and presses the water in the pipeline into the water tank. After the air pump works for Y seconds, the controller sends the signals and the air pump stops. The hot water is discharged, the valve 2 is opened, the water in the auxiliary sealing box flows out and a negative pressure is formed in the sealing box. The water in the water tank is sucked in the auxiliary sealing box, the flow sensor detects the water flow to flow, and the air pump is in a stop state. The valve 2 is closed, the flow sensor does not detect the water flow, and sends the sensing signal. By using the water-saving and anti-freezing system of the solar water heater, normal use of the solar water heater in a frozen season is ensured, water is saved, and the solar water heater is free from the influence of scale formation of the pipeline. The water-saving and anti-freezing system of the solar water heater is operated in the room, when the solar water heater is opened, hot water can be used instantly, and the system is practical and safe.