The geostress in tunnels with high geostress and large deformation under complex geological conditions is often complex and variable. In order to meet the requirements of dynamic design for geostress data, a method is proposed based on the relative ratio of hance horizontal convergence to vault settlement after upper step excavation and the surrounding rock conditions revealed by excavation, then twodimensional numerical model to determine geostress by back calculation is established. Zhongyi tunnel of LijiangShangriLa Railway is taken as an example to explain the method in detail. Research shows that the method described has the advantages of no requiring for geostress test and being convenient to calculate. And it is feasible for geostress analysis and prediction during construction. The back calculation result of geostress in typical sections of Zhongyi tunnel shows that: (1)The distribution characteristics of geostress in the exit and No. 2 transverse tube construction area of the tunnel are different; (2)In the sections surrounding rock and buried depth are similar, the large distinction of horizontal convergence deformation in tunnel is mainly caused by different lateral pressure coefficients.