In order to obtain the flat-top laser beam which has centralized but also uniformly distributed energy required in laser fine processing,the function of the distortion lens used for Gaussian beam diffraction transformation is studied based on diffraction theory,and the analytical formula is derived.Then the principle how the distortion lens makes laser beam flat-top focused is gotten to know.After simulations with the Matlab software,the situations and the effect factors that the distortion lens makes the Gaussian beam flat-top focused are discussed.This indicates that it is feasible to use the distortion lens to obtain the flat-top focused Gaussian beam.The flat-top focused Gaussian beam obtained with this method can be applied in and also play an important role in fields such as laser fine processing,MOEMS and medicine treatment.