A cascaded electro-optical(E-O) switch was studied to suppress Amplified Spontaneous Emission (ASE) and promote the signal contrast of the laser pulse, because the ASE could decrease the signal contrast ratio and widen the waveform and distortion in a high power excimer laser severely. The cascaded mode of the E-O switch was explored, the factors that effect on signal contrast ratio of the E-O switch were analyzed, then the work mode using the E-O switch cascaded was proposed to promote the contrast ratio of the switch. On the basis of a single switch, the cascaded E-O switch layout was designed. The analyzer of the first switch was used as the polarizer of the following one, and the driving power supply was installed discretely to avoid the crosstalk. An experiment on ASE suppression for the low duty cycle of a pre-amplifier was performed using a cascaded double E-O switch. The results show that the contrast ratio of the cascaded switch reaches 104 level and the signal contrast ratio of the pre-amplifier output pulse reaches 106 level, which is beneficial to the amplification of laser pulse signals in following amplifier stages of the system.