Earthquake risk assessment of fault often overlooks the influence of intermediate principal stress (σ2) on fault. However, recent studies have shown that σ2 can alter the internal friction angle, which in turn affects the friction coefficient and strength of rock. In this study, we conducted slip experiments on granite fault to investigate the effect of σ2 on the maximum static friction coefficient (μ0). Our results revealed that as σ2 increased, μ0 also increased, leading to a higher fault strength. Similarly, direct shear tests with varying σ2 yielded consistent observations. The simulation results indicate that an increases in σ2 changes the roughness of fault plane, resulting in variations in the friction coefficient. Furthermore, the fault strength can be described using a true triaxial strength criterion. Our findings emphasize the significance of higher σ2 in the potential risk of major earthquake.