Existence of parasitic oscillation makes the amplifier consume lots of inversed population before the arrival of signal pulse, which reduces peak gain and storage efficiency and influence amplification performance greatly, especially for high-power amplifiers. By pumping Nd∶YAG slab with 8 laser diode arrays, parasitic oscillation in high-power amplifier was studied both theoretically and experimentally. The parasitic oscillations in the slab with different surfaces treatment were compared. When the pump energy was 1.28 J and the input pulse energy was 140 mJ, the single-pass amplification of β=2 and the output pulse energy of 278 mJ was obtained.