Abstract: By using a four-dimensional simulation code,a detailed numerical study of the influence of thermal effects and aberrations in a beam control system on the beam quality is made. The Strehl ratio,β,astigmatism parameters and power in the bucket (PIB) are chosen as criteria for characterizing the beam quality in the far field.It is shown that the defocus,astigmatism and thermal effects result in the distortion of intensity distribution and reduction of the focusability in the far field,thus degrade the beam quality.