Abstract: To investigate the softening characteristics of concrete under different corrosive environments, Mode-I fracture performance tests were conducted in three environments (freshwater, Na₂SO₄, and Na₂SO₄+NaCl). The cohesive toughness was experimentally measured and analytically calculated in various forms, and softening curves at different ages were determined. The results show that the cohesive toughness of concrete in all three environments generally increases first and then decreases. After 270 days, a significant reduction in cohesive toughness is observed with similar rates across environments. At 360 days, the cohesive toughness and crack opening displacement reach their maximum values in the freshwater environment, while their minimum values appear in the Na₂SO₄ environment. This indicates that long-term corrosion leads to concrete deterioration, while the addition of chloride salts helps mitigate the sulfate-induced erosion of concrete. The bilinear softening curve modified by Xu and Reinhardt effectively describes the softening characteristics of concrete in the three environments. When the correction parameters are set to 7, 10, and 8 for freshwater, Na₂SO₄, and Na₂SO₄+NaCl environments, respectively, the measured cohesive toughness values are closest to the analytical values, reflecting the distribution of cohesive force in the fracture process zone of concrete. Analytical values of cohesive toughness derived from the Petersson softening curve are relatively larger. In Na₂SO₄ and Na₂SO₄+NaCl environments, the correction parameters generally decrease with age, ranging from 6 to 10 in the Na₂SO₄ environment and from 8 to 10 in the Na₂SO₄+NaCl environment.