A gas discharge lamp dimming control method aims to provide a dimming period after a start period when a gas discharge lamp is ignited. The dimming period has a duty voltage set initially or corresponding to a remained dimming value in a pervious OFF condition of the gas discharge lamp. Hence the gas discharge lamp is provided with electric energy and maintained in a start condition in the dimming period at any dimming value setting. The duty voltage during the dimming period lasts a length equal to that of the dimming period to allow the gas discharge lamp to gradually restore the required dimming illumination in the start condition and achieve a maximum dimming range.