According to the characteristics of the excavation surface of shield tunnel, based on the analysis of the existing soil limit equilibrium theoretical model of shield tunnel excavation surface, considering the arch effect of soil body under different stratum and different tunnel burial depth conditions, a three-dimensional limit equilibrium theoretical analysis model of unstable soil body is established based on the Paik circular arch theory. Then the classical three-dimensional wedge-shaped body model is improved into a three-dimensional model of unstable soil body with arch effect in both vertical and horizontal planes. Finally, were compared with the centrifugal test and other theoretical models by using MATLAB programming to verify, and the results showed that the results of the mechanical model in this paper are closer to the conclusions of the centrifugal test and can be more accurately applied to the determination of the minimum support pressure parameters at the excavation surface.