Abstract: In order to investigate the effect of principal stress orientation on the stability of the surrounding rock of tunnels, experimental and numerical studies are performed. Cement mortar models are applied, and two cases for the tunnel with and without cracks are considered. Photoelastic experiments are conducted to confirm the results of the cement mortar model tests. In the numerical simulation, the plastic damage of concrete damaged plasticity is adopted. The stresses at the points around the tunnel and the stress intensity factor of crack tips are calculated, and the results are in good agreement with the experimental ones. The results show that for the tunnel without cracks, when the angel of principal stress orientation and tunnel wall θ =45°, the compressive strength is the lowest; for the tunnel with a crack, when the angel of crack direction and tunnel wall β=130°, the stress intensity factor of crack tips is the largest, and the compressive strength is the lowest, and for the tunnel with β=130°, when θ was small or θ=70°, the lowest compressive strength occurs.