The invention discloses a self-discipline rainwater purifying device. The lower end of an L-shaped water inlet pipe is connected with the middle-lower part of a water outlet pipe in a vertical direction. The bottom end of the water outlet pipe is connected with a magnetic return valve. The middle segment of the water outlet pipe is connected with a water storage tank through a pipe. The top end of the water outlet pipe is open. A filter is internally arranged in the water outlet pipe between the junction of the water outlet pipe and the water inlet pipe and the junction of the water outlet pipe and the water storage tank connecting pipe. The device is characterized in that: (1) the device can efficiently use rainwater resources, improves water quality of treated rainwater; (2) installation and operation are simple, high-cost maintenance is prevented; (3) for an existing rainwater drain system of a building, the device is good in adaptability and low in reconstruction cost, and saves space; (4) the device effectively controls a problem of rainwater pollution in an early stage; (5) the device saves energy and reduces emission, and has no energy consumption and no greenhouse gas emission; (6) the device is low in treatment cost, and is beneficial for popularization and application.