An image forming apparatus for generating a stable horizontal sync signal and a horizontal sync signal generating method are provided. The image forming apparatus includes a switching unit to switch on an electric current according to a video signal, a light output unit to output an optical power corresponding to the electric current, which is switched on by the switching unit, an optical power controller to control an intensity of the electric current switched on by the switching unit, and a printer engine unit to output an optical power control voltage to amplify the intensity of the switched-on electric current during a horizontal sync signal generation period included in the video signal. Accordingly, a stable horizontal sync signal is generated, and the deterioration of the quality of a print job is prevented.