In order to study the influence of confining pressure and pore pressure on the mechanical properties and deformation parameters during underground tunnel excavation, a gas-undrained triaxial compression test of layered phyllite was carried out with argon as the permeable medium. The study analyzed the evolution characteristics of stress-strain relationship, peak strength, deformation characteristics and failure modes of layered phyllite with bedding inclinations under varying confining pressure and pore pressure. Additionally, an anisotropy degree formula based on bedding inclinations was introduced to discuss the impacts of confining pressure and pore pressure on the anisotropy of layered phyllite. The results show that the peak strength of layered phyllite increases gradually as the pore pressure decreases or the confining pressure increases. The peak strength c, elastic modulus and deformation modulus exhibit ‘U’ shaped changes with an increase in bedding inclinations. When the bedding inclination ranges between 30° and 60°, the layered phyllite has a low degree of hardness, and the predominant failure mode is shear slip along the bedding plane. Furthermore, the layered phyllite is greatly affected by confining pressure and pore pressure in the direction of parallel bedding, showing significant anisotropic characteristics.