Analysis shows that intensity of the focal spot will be decreased by the systemic optical aberration in a multi-hundred TW ultra-short laser system. In order to increase the intensity of the far-field by avoiding both axial chromatic and spherical aberrations caused by the lens as in the laser system, source of the aberrations was studied, simulation of layout was performed by ray tracing method and air-spaced doublet as the last spatial filter was specially designed to compensate those aberrations. Experimentation proves that the air-spaced doublet can diminish both axial chromatic and spherical aberration of the whole system effectively , the radius of the output focal spot decreases from 8.6 μm to 5.7 μm, and the intensity of the focus increases by eight times.