Based on the analysis of the geometric characteristics of the contour line and plan curve of tunnel crosssections, the author makes further study on the straight line, circular curve and transition curve of the plan curve. Then, the author works out a coordinate back calculation program for those 3 curves by CASIO fx4800 calculator. The mileage and deviation distance are calculated by the mentioned back calculation program. On basis of the calculation program for the overbreak and underbreak, accurate settingout and checking of the tunnel can be performed. This method can also be used for the settingout and checking of other structures of tunnels.