The invention discloses a solar energy water heater with an instantaneous heating function and a self-stabilized output temperature. The solar energy water heater is composed of a water inlet pipe 1, a water inlet valve 2, a water inlet pipe 3, a solar energy heat collector 4, a communicating pipe 5, a water tank 6, a reducing valve 7, a water outlet pipe 8, a quick heating system 14, a water outlet pipe 15, a water outlet valve 16, a water outlet 17, a power source 13, a control system 10, a communicating pipe 18 and a cold water valve 19, wherein the control system 10 is connected with the water tank 6 through a wire 9, is connected with the quick heating system 14 through a wire 11 and a wire 12 and is connected with the water outlet valve 16 and the cold water valve 19 through a wire. When the solar energy water heater works, the control system 10 continuously detects the water level and the water outlet temperature of the water tank 6, continuously detects the water temperatures in the quick heating system 10, the water outlet pipe 15 and the water outlet pipe 17 at the same time, and automatically controls the heating power of the quick heating system 14, so that when the water outlet temperature of the water tank 6 is lower than a desired machine water outlet temperature, the water outlet temperature can be stabilized by real time heating compensation; and meanwhile, the control system 10 controls the cold water valve 19 to open or close, so that when the water temperature output of the water tank 6 is too high, cold water can be supplied to the water outlet pipe 17 through the communicating pipe 18 and the cold water valve 19 and the water outlet pipe 17 can output temperature stabled water by mixing cold water, wherein the communicating pipe 18 is communicated with the water outlet pipe 17.