Summation method with conventional staging to complete the standard U-shaped cross-section surface curve calculation not onlyexist cumulative eror, but also does calculation of the heavy workload with low efficiency.Based on the theory of optimum fitting, with thestandard minimum residual difference as the objective function, in the scope of the engineering applicable parameters, a non-integrable func-tion was obtained by a simplified formula to replace the segments in the original integral. By integration, an analytical model for the cros-sec-tional surlace curve was obtained. Because the positive and negative ftting errors oflset each other the accuracy of the calculation was higher.