The present invention discloses a grinding brush fixing device applying to a pneumatic tool for surface treatment. The spacing between the fixing arms and arc plates of the brush holder is reduced. Thereby, the press-fit face of the axially-extending fixing arm can press the brush collar onto the first press-fit edge and second press-fit edge, which are at two sides of the notches between two axially-extending arc plates. Further, a flexible gasket is radially placed in a space between the fixing arms and the arc plates. When the fixing disc is fastened to the brush holder, the flexible gasket and the brush collar are tightly compressed and firmly secured. Thus, force can be uniformly applied onto the brush collar and the bristles. The present invention can promote the stability of brush's rotation, generate straighter ground strips, prolong the durability of the grinding brush and grinding brush fixing device, and promote the grinding efficiency.
