Optimal crystal length, energy conversion efficiency and pulse width of output signal light of a CsLiB6O12 optical parametric amplifier that is pumped by an ultra-short high power laser with output wavelength of 355 nm and pulse width of 10 ps are theoretically analyzed and numerically simulated. It is presented that CsLiB6O10 crystal optimum length is 27.0 mm in the case of Ip(0)=300 MW/cm2, Is(0)=5 MW/cm2, Ii(0)=0. With increasing the pump power and signal power, CsLiB6O12 crystal length is shorter when it acts as nonlinear crystal of optical parametric amplifier. The signal power output of high conversion efficiency is depended on the optimum crystal lengths, pump intensity and signal power.