Abstract: In order to find out the creep behavior of salt-gypsum under the down hole conditions,the mechanism affecting salt-gypsum creep was studied.By adjusting the concentration of salt in the salt solution to control the relative humidity around the rock samples,the uni-axial compression test was carried out at 10 MPa and tri-axial compression tests were carried out at confining pressures of 0,2 and 5 MPa under relative humidity of 35%,65% and 95% respectively.It was found that in a short-term test of salt-gypsum rock,the rock strength displays a downward trend from 38.2 MPa to 35.2 MPa with the increase of relative humidity from 30% to 90%,the change in Young’s modulus and Poisson’s ratio is small.When the relative humidity is 95%,the sample generates negative volumetric strain (expansion) and when the relative humidity is 35%,the sample generates positive volumetric strain (shrinkage).The study shows the strength of salt-gypsum is not highly sensitive to the change of relative humidity;and combined with the Heard model,the impact of relative humidity on the volumetric strain rate can be explained by the theory of creep activation energy;and the influence of relative humidity on the volumetric strain can be explained by the mechanical-chemical mechanisms of mutual migration of water molecules between the crystal and capillary spaces.The creep behavior of salt-gypsum rock underground revealed by the study can provide theoretical basis for safe drilling in this kind of formations.