The rheological model of rock salt under cyclic loading is one of important contents of rock mechanics. Here, according to the actual cyclic loading fatigue process of rock salt samples, rock salt’s strain versus time constitutive equation within initial uniform velocity loading stage and sinusoidal cyclic loading one was deduced based on Poyhting-Thomson model. The cyclic loading fatigue tests of rock salt were conducted. Due to fatigue damage, the average strain increases at equal speed with increase in time. Accordingly, the theoretical constitutive equation was modified, and the power function of time was used to describe the strain increment to obtain the modified Poyhting-Thomson fatigue rheological constitutive equation. In uniform velocity loading stage, the theoretical model was able to simulate the test curve well. The modified rheological constitutive model was used to fit the upper limit line, lower limit one, and middle one of rock salt’s fatigue strain curve in cyclic loading stage to get corresponding fitting parameters. Then taking the average values of fitting parameters, the modified model was used to simulate the   fatigue test curve, the simulated curve agreed well with the test one and it could almost replace the test one. Test and simulation showed that Poyhting - Thomson model and its modified model are feasible to describe rock salt’s fatigue strain development under cyclic loading, they can better depict the law of rock salt’s strain versus time.