The ratio of the signal to noise of a high power laser system is the ratio of the main pulse to the noise before it. Previonsly, when a photoconductor switch and an oscillograph were used to measure the ratio of the signal to noise up to ～ 106, the oscillograph was strongly striked by the main pulse. With an improved method, same measurement result was obtained while the main pulse amplitude was greatly reduced, the measuring instrument is thus effectively protected.