Lining structures are subjected to three-dimensional stress transferred from the overburden soil and rock formation and usually encounter external sulfate attack; permeability and mechanical properties of cement materials are significant for the stability of underground engineering. Geotechnical testing methods were adopted to analyse the evolution of gas permeability, ultrasonic P-wave velocity, uniaxial compression and splitting tensile strength of cement mortar specimens subjected to sulfate attack. With increasing confining pressure, the gas permeability gradually decreased, and the rates and amounts of decrease for the attacked specimens and non-attacked specimens were different. The gas permeability of attacked specimens was greater and more sensitive to the variation of confining pressure ranging from 5 to 20 MPa. Under high confining pressures ranging from 20 to 30 MPa, the rate of decrease in gas permeability was reduced. The ultrasonic P-wave velocity of attacked specimens was visibly lower than that of non-attacked specimens, while their open porosity was higher than that of their counterparts. Both the splitting tensile strength and uniaxial compressive strength decreased after sulfate attack, and the deterioration of the former one was more severe. The experimental results can provide reference material for establishing the long-term stability of concrete structures in a high-sulfate environment.