Abstract: The method of Gaussian beam shaping using an aspheric lens has been very mature, and specific aspheric shaping mirrors can be designed according to the incident light parameters. However, when the aspheric shaping mirror designed with a 3 mm incident beam waist is tested, it is found that the aspheric shaping mirror is not only applicable to the incident parameters in the design. When the incident beam size and divergence angle are different, there will be an optimal shaping position with flat top distribution behind the shaping mirror. The position is far away from the shaping mirror with the increase of the diameter of the incident beam, and close to the shaping mirror with the increase of the divergence angle. In order to explore the difference in shaping results on the optimal shaping position, the control variable method is used to carry out the experiment. It is found that the diameter and divergence angle of the incident light beam have no obvious change in the flat factor of the flat top distribution on the position, but the beam uniformity and edge steepness will have the best value, and there is an optimal incident parameter. In order to obtain the relationship between the optimal shaping position and the incident beam diameter and divergence angle, a mathematical model is successfully established by using the response surface method. When the beam diameter and divergence angle at an incident position are known, the optimal shaping position can be quickly obtained.