A fixing device for detachably fastening an electronic element inside thereof is described. Two fastening modules are respectively disposed on the two opposite inner walls of a housing. A positioning disk is rotatably connected with the positioning frame and has a concave section with a pair of inner side surfaces. The positioning disk is disposed between the first pair of positioning surfaces while the positioning disk is disposed between the second pair of positioning surfaces. When the positioning disk is at a first position, the pair of inner side surfaces is aligned with the first pair of positioning surfaces to allow the electronic element to be inserted between. When the positioning disk is rotated from the first position to the second position, the pair of inner side surfaces is aligned with the second pair of positioning surfaces, and the first and second connection members engage each other to fasten the electronic element.