Abstract: It was somewhat difficult to use mathematical models of general polynomial to fit the silk size curve accurately,because the shape of the silk size curve was diverse.To improve precision in fitting the silk size curve,the paper proposed a new way to fit the silk size curve based on the analysis of the characteristics of the silk size curve.The silk size curve was first detached into two parts: the liner formation and the nonlinear formation,then the liner formation and the nonlinear formation were fitted respectively.The linear part was fitted directly using the linear function and the nonlinear was fitted by harmonic analysis and the least squares method.Good results were achieved.In order to make the silk size curve of fitting more practical in actual production,it advanced standardized methods of silk size curve and prediction method,that is,symmetric extension method,to estimate size values of both ends of the silk.