The model that the laser beam with random phase aberration propagates through the beam smoothing phase plate has established.Using the random theory,the characteristics of the transmission light from the phase plate were studied.And the statistic mean value and the variance of the far field distribution were calculated.The analysis results show that the relationship of the phase plate’s Fourier spectral T(f)2 and the far field distribution variance is in direct ratio.That means with the increase of the width of T(f)2 the far field distribution of the beam become smoother.The stimulated annealing method was employed to design two different phase plates for beam smoothing.And the numerical results approve the theoretic conclusion.