In the process of shield tunneling, the longitudinal displacement caused by the uneven jacking force of jack is one of the main reasons for the initial dislocation between segments. The calculation method of tunnel longitudinal displacement based on Timoshenko beam theory fails to consider the deformation characteristics of unloading failure of surrounding rock after tunnel excavation and the synergistic effect of surrounding rock and lining structure, so the prediction error is large. The calculation method of equivalent foundation resistance coefficient considering the unloading and expansion effect of surrounding rock is established, and the calculation formula of equivalent shear stiffness considering the number of bolts is put forward. An improved theoretical calculation method of longitudinal displacement of segment-assembled tunnel based on Timoshenko beam theory is established. The analysis of practical engineering cases shows that it is more reasonable to consider the nonlinear volume-dependent plastic deformation (i.e. dilatancy effect) of surrounding rock during excavation and unloading. Considering the timeliness of the curing process of grout behind the wall, the maximum longitudinal displacement calculated by the improved sliding boundary condition is 2.3 times that of the traditional fixed end boundary, and the boundary condition has a great influence on the calculation results. The calculation method in this paper is more in line with the actual situation and safer.