Abstract: The influence of random-distributing structure surfaces in rock mass on the soft rock rheology is mainly expressed by scale effect. Multi-step incremental cycling loading and unloading uniaxial compression creep tests on 4 different sizes of sand-shale specimens showed that soft rock rheology could be described by the Salustowicz model and the values of its constitutive parameters of E H(linear elastic parameter), E_K(viscoelastic parameter) and η_K(viscous parameter) would decrease and tend towards being constant as the specimen size increased, which might be expressed by nonlinear regressive equations with extremum conditions. Then an extrapolation method was gained to determine the dimension of the continuous micro element of engineering rock mass with randomly distributing structure surfaces. It may be used to gain the rheological model and its mechanical parameters of engineering rock mass in the continuity schematization range by rheological tests on the different sizes of rock specimens in the laboratory.