Abstract: Numerical simulation has been carried out to study the evolution of perturbed laser beam after passing through a piece of nonlinear medium.The variation of "hot image"with incident beam energy,modulation depth and modulation width have been studied.It has been concluded that:in some conditions,there can be a "hot image" at the ordinarily called "conjugated plane",but in other conditions,peak fluency density is very high at some places other than "conjugated plane".Phase modulation depth can deeply influence the evolution of the laser beam,and some kind of depth may induce more serious damage than others.In addition,modulation width and incident beam energy will also influence the distribution of the field.