In order to improve the operation security of Shenguang II 5 PW (SGII-5 PW) ultrashort pulse laser system, the near-field distribution uniformity of large energy optical parametric chirped pulse amplification (OPCPA) beam is numerically simulated, and the simulation results are compared with experimental results. In the 1 PW amplifier, based on the measurement data of the preamplifier and the SGII high energy pump pulse, the evolution of the near field fill factor and the fluence beam contrast in optical parametric amplification process are presented by numerical simulation of parametric coupling wave equations. The conversion efficiency and the amplification stability are also discussed. The optimal range of nonlinear crystal length, in which high beam quality, high conversion efficiency and high stability can be achieved simultaneously, is obtained. The results also indicate that the pump light greatly affects the amplified beam uniformity. Enhancement of the SGII 7th beam quality is a practical way to improve the beam uniformity of second stage OPCPA.