A fixing structure for fixing a reflector and a device for detecting a display brightness are provided. The fixing structure is configured to fix a reflector to a photosensitive detecting assembly; and the fixing structure includes at least one clamping unit configured to clamp and fix the reflector. Each clamping unit includes: a first support, including a first clamping arm configured to support a first surface of the reflector; and a second support, including a second clamping arm configured to press on a second surface of the reflector facing away from the first surface.