Abstract: The output average power of all-solid-state picosecond amplifier suffered from self-focusing effect existed in the gain crystals. Through introducing the compensation element-gallium arsenide(GaAs)plate, the damage due to self-focusing effect has been avoided and the suppressing mechanism has been studied through theoretical analysis and experimental research for high peak power Nd:YAG crystal picosecond amplifier systems. The nonlinear refractive index coefficient of GaAs was obtained by calculation, and the relationship between the thickness of GaAs plate and the length of Nd:YAG rod under the optimal effect of suppressing self-focusing was given by numerical simulations. Under the condition that the center wavelength of the incident picosecond laser beam is 1 064 nm, the repetition frequency is 1 kHz, and the peak power density is about 12 GW/cm2, experiments on the effect of GaAs plates with different thicknesses(200 m and 550 m) to reduce the self-focusing damage in Nd:YAG rod have been completed. With optimization of thickness of the GaAs plate, the compensation method demonstrates high efficiency under high peak power picosecond pulses condition especially for Nd:YAG amplifier.