A kind of device supplied with negative pulsed high voltage was designed for water treatment.The nozzle discharge electrode was supplied with high voltage and the cylinder wall of the treatment chamber was grounded.Since the non-thermal plasma,especially,hydroxyl radicals played the dominant role in removing pollutants in the water by discharge plasma,hydroxyl radicals were measured in the negative pulsed discharge process by the optical emission spectroscopy(OES)effects of the repetition rate and the peak value of the pulsed discharge on the hydroxyl radicals production and the decomposition rate of the Acid Orange Π solution were studied.