In the pre-amplifier of high power laser facilities, more than 106 gain is produced by the high gain amplifier, in which self-excitation oscillation would be easily formed. Based on the theory of self-excitation oscillation, the gain and loss in the system are analyzed, and the self-excitation problems in the high gain amplifiers of TIL of the SG-Ⅲ facility are studied. There self-excitation cavities would possibly exist in the system. Using the methods including tilting the ends of Ndglasses, adjusting the beam, utilizing isolations and electro-optical switches, the self-excitation oscillation is controlled. These suppression methods are validated in experiments.