A projection device including a projection unit, an image capture device and a control unit is provided. The projection unit includes a light source. The projection unit is configured to project an image picture to a projection area. The image capture device is configured to obtain an image capture picture including the image picture in an image capture range. The image capture range is larger than the projection area. The control unit is coupled to the image capture device. The control unit is configured to determine, according to a picture variation value of the image capture picture, whether to adjust the brightness of the light source. In addition, a brightness adjusting method used for the projection device is also provided. According to the disclosure, the brightness of the light source of the projection device may be properly adjusted according to environmental factors.