A discharge lamp lighting circuit includes an open protection circuit. The discharge lamp lighting circuit includes an open protection circuit and an over-voltage detecting circuit. The open protection circuit detects whether any of the discharge lamps are unlit, and sends a voltage control signal to a control circuit if one of the discharge lamps is not lit, whereby the control circuit sends a pulse signal to a buck converter according to the voltage control signal, and the buck converter generates and outputs a DC voltage according to the pulse signal. If the DC voltage from the buck converter reaches a predetermined voltage, the over-voltage detecting circuit conducts and sends a voltage control signal to the control circuit, and the control circuit ceases operating according to the voltage control signal. Consequently, damage to the other normal discharge lamps is prevented.