In order to study the effects of cement-lime compound modification of expansive soil on water quality, expansive soil samples were taken from the Project of Water Diversion from the Yangtze to Huaihe River which were modified by cement, lime, and cement-lime. The triaxial specimens were fabricated and conditioned for different ages(1 d, 3 d, 7 d, 28 d). The specimens with different curing ages were immersed in water for 1 d, 3 d, 7 d, 28 d, after which the leaching solution was taken and subjected to pH value and Ca⁺ content tests. By comparing the leaching solution testing results and observing the integrity of the specimens after immersion, whether compound modification can reduce the soil impact on water quality was explored. It is found that the pH value and Ca⁺ content of the leaching solution from compound modified soil samples are lower than that of lime modified samples, and the compound modified soil with 28 d curing age has less effect on water quality than that of cement modified soil. In addition, it is suggested a curing age of at least 28 d should be guaranteed in practical engineering, during which rain wash or long-time water soaking should be avoided to ensure the curing quality.