Abstract: Based on the actual adjustment path of the surrounding rock stress in the process of tunnel excavation, the grading unloading rheological tests under unloading confining pressure with axial compression are designed, and the unloading rheological mechanical properties of sandy mudstone which is mainly threaded by the tunnel axis of diversion tunnel of a hydropower station are studied. The test results show that: (1) When the stress level is lower than the rock failure stress, the rheology of sandy mudstone only has the stages of attenuation creep and steady-state creep, and with the unloading of confining pressure step by step, the stress level is gradually over the long-term strength of rock, and the steady-state creep rate of sample gradually increases from being close to 0 originally to a constant greater than 0. (2) When the stress level is higher than the rock failure stress, the sandy mudstone goes into nonlinear acceleration creep stage until destruction after the stages of attenuation creep and steady-state creep. By comparison, the whole time of the sample for rheological damage under different initial unloading confining pressures is different, and the start time of the sample for accelerated nonlinear rheology is also significantly different. The analysis of the test results shows that Nishihara model can well describe the rheological properties of sandy mudstone when the stress level is lower than the failure stress, but it cannot reflect the nonlinear accelerating rheological characteristics when the stress level is higher than its failure stress. Based on this, by introducing a start element of nonlinear acceleration creep of rock and combining with Nishihara model, a new nonlinear visco-elastoplastic rheological model is established. The rheological parameters of the model are identified, and the fitting results of the rheological model are compared with the test results. It is concluded that the proposed unloading rheological constitutive model can fully