Accurate prediction of overlying soil pressure is of great significance to the design of tunnel support structures and the selection of excavation methods. Terzaghi’s theory of the soil arch effect is established based on the assumption that the slip plane is vertical， but in practice， the slip plane shows a curved shape due to the formation disturbance. To study the evolution law of slippage surface of cohesive soil layer and the distribution law of loose earth pressure， firstly， the overlying soil pressure after tunnel excavation is calculated by numerical simulation software， and the evolution law of the soil arch effect under cohesive soil is analyzed. Secondly， based on the fracture surface of the tunnel arch， the ellipsoid theory and the circular arc of the principal stress in the cohesive soil layer are used to correct the Terzaghi loose earth pressure. Finally， the theoretical calculation results are compared with the existing experimental data and the finite element calculation results to verify the rationality of the application of the formula in the cohesive soil layer， and to further study the relationship between formation loss rate SL， internal friction angle φ， cohesion c， and tunnel loose earth pressure. The results show that the damage degree of slippage surface in the cohesive soil layer is greater than that in the non-cohesive soil layer. Still， the changes in slippage surface in the cohesive soil layer are the same. When the buried depth ratio of the tunnel is less than 1.5， a triangular slip plane appears. As the buried depth of the tunnel continues to increase， a shear plane is gradually formed inward， and a tower-shaped slip plane is finally formed. The lateral earth pressure coefficient Kv inside the loose zone is different at every location and fluctuates around 1.0， as suggested by Terzaghi. Compared with shallow buried tunnels， the loose earth pressure of deep buried tunnels is more affected by formation loss rate SL. The overlying load of the tunnel in the cohesive soil layer is distributed in a “half gourd shape”， which gradually decreases from near the vault to the arch waist. At the same time， in the formation with a small internal friction angle φ， increasing cohesion c is beneficial to reduce the overlying soil pressure of the tunnel.