To investigate the acid corrosion-induced deterioration law of mechanical properties of cement mortar under coupled seepage and salt attack, cement mortar erosion tests, uniaxial compression tests, and splitting tensile tests were carried out under different acidic conditions (pH=7, 3, and 1) in Na₂SO4 solution with seepage. The deterioration mechanism of the mechanical properties of cement mortar under the coupled seepage and salt attack under different acidity conditions was revealed, and a deterioration analysis model for the mechanical properties of cement mortar was established. The results show that at pH of 7 and 3, the compressive strength and tensile strength of cement mortar specimens initially increase and then decrease with increasing erosion duration, reaching their peak values at 60 d and 45 d, respectively. Specifically, the peak compressive strength and tensile strengths are 39.33 MPa and 3.12 MPa at pH of 7, as well as 36.71 MPa and 2.86 MPa at pH of 3. In contrast, at a pH of 1, the compressive strength and tensile strength of the cement mortar specimens decrease, with their maximum values being 31.00 MPa and 2.30 MPa, respectively. A higher acidity of the erosion solution indicates a lower peak compressive strength and tensile strength of cement mortar and a more pronounced deterioration effect of mechanical properties. Under coupled seepage and salt attack, as acidity increases, the deterioration of cement mortar shifts from being dominated by the coupled action of leaching erosion and sulfate attack to being dominated by acid erosion. Under the coupled seepage and salt attack, the strength damage coefficient of cement mortar exhibits a good quadratic function relationship with erosion duration, and there is a linear correlation between tensile strength and compressive strength.