A theoretical study is conducted on the instability of the unsupported face and the variation of overlying earth pressure induced by shield disassembly in soft soil strata, aiming to reveal the mechanism of the soil arching effect under complex conditions and provide theroretical guidance for shield docking and disassembly construction. Based on a shield tunnel in the soft soil strata of Shanghai, this study analyzes the soil arching effect in the ground under two cases: without reinforcement and with reinforcement in the docking section. Based on the formation characteristics of the soil arch slip surface in the docking section, corresponding soil arch mechanical models are established. Furthermore, by incorporating the distribution of major principal stress trajectories, a formula for calculating the overlying earth pressure on the tunnel in the docking region is derived. The rationality and applicability of the established model are verified by comparing the theoretically corrected solution with numerical simulation results. The findings reveal that, without grouting reinforcement, the soil arch slip surface extends obliquely upward from the tunnel to the ground surface, forming a basin-shaped failure surface in both transverse and longitudinal sections, with a wider range in the transverse direction. After grouting, the transverse slip surface exhibits either a "double-basin" or "basin-over-cone" pattern, while the longitudinal slip surface forms a conical shape within a limited range above the tunnel. Additionally, vertical stress decreases within the slip surface and increases outside it. Moreover, the difference between the average vertical stress and soil self-weight increases progressively with depth. However, in non-grouted conditions, the vertical stress curve remains smooth, whereas grouting reinforcement causes a distinct inflection point in the curve. Finally, without grouting reinforcement, the soil resists the relative displacement along the slip surface induced by shield disassembly mainly by mobilizing the friction angle along the slip surface. After grouting reinforcement, stress redistribution occurs within the soil, and the soil arching effect is significantly weakened.