Model of shield tunnel considering the shearing dislocation was used to investigate the effect of the new shield tunnel undercrossing on existing tunnels. In light of the fact that the additional stress along the central axis of the existing tunnel was caused by the bulkhead additive thrust, the friction force between shield and soil, and grouting pressure in the process of undercrossing shield tunnel construction, the lining ring of shield tunnel was regarded as a series of short elastic foundation beams connected by shear spring. An equation was developed by the principle of minimum potential energy, and three-dimensional change process of the vertical deformation, shear force and dislocation between rings with the excavation of the new tunnel were worked out. The research shows that the vertical displacement value calculated by the proposed method combined with the shearing dislocation model and the principle of the minimum potential energy agreed well with field test data. The dislocation of the segments where the vertical displacement is the maximum is close to 0. The maximum value of segment ring's dislocation and shearing force appears at the inflection point of vertical displacement curve. With the excavation of the new tunnel, the vertical deformation, shear force and dislocation between rings of existing tunnel increased and finally stabilized.