Triaxial rheological test was carried out for marble from Jinping Hydraulic Power Station under step unloading confining pressure with constant axial stress on the basis of the engineering practice. According to the experimental results the creep law of axial and lateral deformation under unloading stress path was studied. The irreversible deformation property of rock in the process of unloading rheological test was also analyzed. Some conclusions were made from the test and the analyzed results show as follows：(1)Unloading confining pressure affects creep deformation and instantaneous deformation of rock. (2)Development of lateral irreversible deformation is more rapid than that of the axial direction in? comparison with the moment of unloading. (3)The response to failure in lateral direction is more obvious than that in axial direction. Through research of the test the unloading creep law of the marble is grasped, and meanwhile the test provides important test data for following theory model research.