We designed a binary optic element (BOE) applying in the excimer laser wavefront shaping, which can obtain an optical field of Butterworth distribution. We employed the Gerchberg-Saxton (GS) algorithm for realizing the optimization and MATLAB was utilized to simulate the change of optical field. By comparing the simulation results of iterative number with 10, 100 and 1000 respectively, we studied the impacts of iterative number on shaping effects in Gerchberg-Saxton algorithm. Through the shaping result by cimulating about 106 time′s iterations, the phase distribution of binary optic element was obtained. The results present that the distribution of emergent optic field is Butterworth whose energy is 75.62% of total energy and the root mean square (RMS) of uniformity is 0.1394%.