Based on the nonlinear Schrödinger equation and Fourier transformation, the intensity distributions of the high power flat-topped beam propagating in the nonlinear medium and the free space are studied. The influences of the defects on medium surface on the intensity evolution of flat-topped light beam in the medium and the free space are analyzed. The results show that the larger the beam order, the poorer the beam self-focusing property is, and the closer to the back surface of the medium the position related to the maximal intensity point is a stronger initial intensity, a thicker medium or a higher nonlinear coefficient will lead to a stronger self-focusing property. The defects on the surface of medium will produce a relatively large intensity, closer to the back surface of medium. The effect of the phase modulated defect on the intensity evolution is greater than that of amplitude modulated defect.