The invention discloses a power automatic adjusting device and method for a winnowing machine. A feeding vibration groove is arranged at the front end of a feed inlet of the winnowing machine, the front end of the feeding vibration groove is connected with an electronic belt scale, an infrared moisture meter is arranged near the feeding vibration groove, a detecting portion of the infrared moisture meter is aligned with the feeding vibration groove, an air inlet is formed below the feed inlet of the winnowing machine, the upstream pipeline, connected with the air inlet, of an air return pipeline is provided with a positive pressure fan, the tail end of the air return pipeline communicates with an air outlet of the winnowing machine, the downstream pipeline close to the air outlet is provided with a negative pressure fan, the position, close to the air inlet, of the bottom of the winnowing machine is provided with an impurity outlet, the position, away from the air inlet, of the bottomof the winnowing machine is provided with a discharge gas lock, and the electronic belt scale, the infrared moisture meter, the positive pressure fan and the negative pressure fan are all connected with a controller. The power automatic adjusting method comprises the steps of setting of basic materials, collection of real-time data and automatic adjusting. The power automatic adjusting device andmethod can adjust wind power according to practical conditions so as to achieve precise separation of cut tobacco and impurities. The device is simple in structure, and the method is easy and convenient to operate and easy to implement.