High quality far-field determined by the phase and amplitude distribution of focal beam is necessary for inertial confinement fusion (ICF). To the high power laser system, the amplitude distribution of the output laser beam mainly derives from input beam, while phase distribution mainly come from the large aperture optical elements, especially for the Nd:glass amplifier slab. Nonlinear self-focusing of ripple can effect on far field in high energy too. Based on these factor, the transmit character of high power laser with fourth-pass amplifier was simulated, and the effect of amplitude and phase distribution on far field was analyzed by numerical was. The conclusion is that the most important effect factor on far field is phase distortion come from large aperture optics.