In high power excimer laser system, the signal contrast ratio is decreased severely by the amplified spontaneous emission (ASE), which decreases the laser pulse signal contrast severely, leads to waveform broadening and distortion, and impacts on the accurate physical experiment. In this paper, based on the principle of the short pulse generation by electro-optical switch, study on ultraviolet electro-optical switch is made, and a method for ASE suppression of the laser amplifier chain is established. In the waveform clipping experiment of the first pre-amplifier in master oscillator power amplifier (MOPA) system, the extinction ratios of the single and cascaded dual electro-optical switches reach 103 and 104 level, and the laser pulse signal contrast ratios are promoted to 105 and 106 level after clipping, respectively. In the experiment of single channel MOPA system, the cascaded dual electro-optical switch is adopted to control the ASE, and an amplified narrow pulse is obtained on the target surface, providing an effective solution to the problem of waveform amplification in the high power excimer laser system.