The invention discloses a method and device for adjusting image brightness, and belongs to the field of image processing. The method comprises the steps: carrying out the guide filtering of a to-be-processed first image, and obtaining the background brightness image of the first image; carrying out the brightness balancing of the first image according to the background brightness image, and obtaining a second image with the uniform brightness in all regions. The device comprises a guide filtering module and a brightness balancing module. The method can enable the brightness of all parts of a photographed image to be uniform.