Abstract: Using rock mechanics test system with MTS815 Flex Test GT, the high pore water pressure effects of mechanical properties of the sandy slate rock mass are studied through several stages of tests, such as simulation of the stress state of rock mass, mechanical properties tests of rock mass without water pressure and coupling tests of water-rock. The results and analysis show that the strength and deformation properties of the sandy slate rock mass decrease with the increase of the water pressure. The water pressure has the greatest impact on cohesion c, the least impact on angle of internal friction φ, and a moderate impact on deformation modulus. The cohesion creduces sharply with the rising water pressure. When the water pressure gets to a high level, rock mass completely loses cohesion. Both the deformation moduli E₅₀ and E₀ decrease with the rising water pressure, and their variation gradients have little diffience. These conclusions reveal the effect of pore water pressure on the mechanical properties of the sandy slate rock mass, and a prediction model for the main mechanical parameters is established. This study is valuable to solve practical engineering problems.