Regarding the fitting method in the crosssection monitoring of Metro tunnels, least square method and selecting weight iteration method are presented. The calculation procedures for selecting weight iteration method are realized on basis of MATLAB. The two fitting methods mentioned above are compared on basis of the monitoring data of Line 10 of Nanjing Metro. Conclusions drawn are as follows: Without outliers, Both the least square method and the selecting weight iteration method have high fitting precision; With the increase of the number of outliers, the least squares fitting data become to be distorted and the outliers cannot be found correctly; The outliers can be found correctly by selecting weight iteration method and the monitoring points without outliers have higher fitting precision.