The invention discloses a brightness adjusting device for a beacon light. The brightness adjusting device comprises a beacon light holder, a light-emitting device located on the beacon light holder, arotating device located at the bottom of the beacon light holder and used for driving the light-emitting device to rotate, and an external power supply used for providing power for the light-emittingdevice. The light-emitting device comprises a plurality of light-emitting surfaces. The brightness adjusting device comprises a processor used for setting the brightness information of the light-emitting surfaces and adjusting the brightness of the light-emitting surfaces, a sensing device used for collecting the position information of the light-emitting surfaces and sending signals to the processor, and a signal element used for sending the position information of the light-emitting surfaces to the sensing device. When the sensing device receives a signal sent by the signal element, the sensing device sends a signal to the processor. The processor adjusts the brightness of the light-emitting surfaces according to a brightness value set by a user. When the sensing device cannot receive any signal sent by the signal element, the processor restores the brightness of the light-emitting surface to a normal value.