An optical storage apparatus and a method for automatically adjusting a loop gain thereof are provided. The method includes the following steps. Firstly, a compact disk is written by an erase power. Next, a writing state of the erase power is detected to generate an error signal. Finally, a loop gain of a servo control loop is corrected according to the error signal. Therefore, the loop gain can be corrected by the present invention according to a reflectance of the compact disk.