Abstract: Salt rock deposits in China are mainly of layered structures.During the construction of underground salt cavern gas storage by means of solution mining, a large amount of insoluble is accumulated at the bottom of the cavity, and consequently the effective storage volume is decreased and the cavity building cost is increased.To calculate and evaluate the actual volume of insoluble in salt rocks at the bottom of the cavity, the swelling rates of insoluble at different calcium sulfate mass fraction, different calcium sulfate type, different temperature and different particle size were studied by means of chemical experiment method.And the experiments are based on the JTG E40-2007 Test Methods of Soils for Highway Engineering which is applicable to soil swelling tests.It is shown that the swelling rate of insoluble in salt rock is negatively correlated with the content of calcium sulfate and positively correlated with its particle size and the temperature.In addition, it is also associated with the type of calcium sulfate, and it is more affected by the content of calcium sulfate dihydrate than that of calcium sulfate anhydrate.When the content of calcium sulfate dihydrate is equal to that of calcium sulfate anhydrate, the effect of temperature on the swelling rate of insoluble is more obvious in the case of calcium sulfate anhydrate.The research results are of great significance to the solution mining of underground salt cavern gas storage.