In the laser direct metal deposition forming, surface flatness is the key to its forming quality. To obtain the ideal surface flatness, ER-2209 wire was selected, and the weld seam was shaped based on a single-layer, single-pass fusion weld by laser side-axis feeding. MATLAB software was used for image processing, seam feature extraction, and mathematical fitting. The experimental results show that the parabolic model of the conventional weld differs greatly from the characteristics of the actual clad weld. In order to ensure that the error is controllable and at the same time avoid the generation of unfavorable factors, after several groups of fitting results of different orders are compared, it is found that the use of the 6th order function is a better fit for the experimental object. The optimal fitting function can be derived from the fitting results of the weld curve, and then the best theoretical lap rate can be solved by using the idea of differentiation to establish the mathematical model of optimal flatness. The surface flatness model proposed in this paper was verified by 5 sets of single-factor laser side-axis wire feeding lap experiments. The results show that the weld surface flatness is a function of weld characteristics and interlayer offset, and the theoretical optimum lap rate of the model is 30.46%, ideally between 30% and 35% for experimental verification, the theoretical optimum lap rate holds within the allowable experimental error. In the actual laser side axis feed wire fusion forming process, small flexible adjustments based on the theoretical optimal lap rate can be made according to the actual situation.