<p>Driving the shield machine at a longitudinal inclination angle is necessary in complex geological conditions, to avoid existing underground structures as well as at entry and exit from the ground surface. This paper investigates the face stability of inclined shield tunnels using a refined 3D Finite Difference Method (FDM) model. The strain-hardening behavior of the muck is captured by the UBCSAND model. The actual mechanical response of the muck is simulated through a two-phase approach, which is subsequently employed to simulate the progressive collapse of the excavation face. The parametric analysis based on the refined FDM model is carried out over a range of effective friction angles, cohesion values and longitudinal inclination angles to evaluate their effects on the excavation face stability. A dimensionless equation for predicting the critical earth chamber pressure is proposed, which shows good agreement with the model test. Finally, a case study on the Ultra Rapid Under Pass (URUP) tunnelling project in Nanjing, China is chosen to verify the proposed 3D FDM model.</p>