Abstract：The construction of underground engineering often encounters soft rocks，whose compressive strength is lower，with obvious characteristics of rheology. The amount of creep deformation is big enough to cause failure of support. The characteristics of rheology should be taken into consideration for analysis of stability or design of underground engineering. Triaxial creep test is made for mudstone with the authors-developed gravitation lever style rock creep test machine and triaxial pressure cell. Testing results show that the creep of mudstone is of nonlinearity. On the basis of testing results，nonlinear creep equation is established. According to the equation，the stress and displacement field of surrounding rocks may be analyzed. Theory research result shows that the basic way to control excessive creep deformation is to improve the stress state of surrounding rocks，and properly increase prestress of bolt or cable，so that the theory basis is provided for effective control of the harmful deformation of surrounding rocks in deep mining.
