Abstract：Large rheological deformation of softening sandstone at great depth is exclusively associated with perturbation. And the study of rock rheology and its perturbation effect becomes especially important. Two new concepts，limit neighbor of rock strength and perturbation effect of rock rheology，are proposed. An experimental result is obtained that the perturbation effect is obvious when the rock sample is in the scope of limit neighbor of rock strength，which indicates that the rock rheology and its perturbation osculate with the rock¢s strain condition；and the perturbation effect is sensitive to the limit deformation. With the concept of limit neighbor of rock strength and perturbation effect of rock rheology，an alternative experimental instrument is developed for studying such a perturbation effect，and it is also used to test some sandstone samples under the conditions of constant axis-loads at different rheological phases. Some perturbation time and cumulative deformation curve of sandstone rheology at different phases are achieved for studying the rule of rock rheology and its perturbation effect. Finally，the factors of perturbation effect and a rheological constitutive equation for sandstone at different rheological phases are presented to simulate the perturbation effect. The constitutive equation can be used to calculate the rheology and its perturbation increment of the same rock；and the stability of surrounding rock can be analyzed.
