There are numerous control parameters of metal surface marking by laser which providing great influence on image quality. Pulse fiber laser is used to mark the two-dimensional code on Al99.70 aluminum ingot surface. The optimal mathematical model is established between processing parameters and gray value. The interaction between related processing parameters is analyzed, the process parameters are simplified and integrated. The single factor experiment is carried out with the simplified parameters, and the reasonable parameter range is obtained through the analysis of both distribution and change law of gray value of marked image. Finally, based on the results of single factor experiment, a multivariate nonlinear regression mathematical model between process parameters and gray value is put forward, and then the multi-factor orthogonal experiment data and immune clone algorithm are utilized for the fitting of model parameters. The effectiveness of mathematical model is demonstrated by verification experiment which provides the theoretical basis for the realization of laser two-dimensional code marking on aluminum ingot surface.