Experimental investigations of nonlinear shear viscosity of subgrade soils under periodic loading are conducted. Unsaturated subgrade soil samples are prepared to conduct triaxial shear tests under repeated loading that is used to simulate the cyclic stress induced by vehicles. Investigations included three aspects. First，a new nonlinear viscous model is introduced to describe the relation of shear stress and shear strain rate. This relation was reduced from the constitutive law of nonlinear poroviscosity suggested by the author before. Second，related constitutive parameters are investigated and calibrated with experimental results from triaxial shear tests. The nonlinear viscous parameters are assumed to be a function of deviatoric strain and loading repetitions. Third，both viscous and elastic behavior of soils associated with the deviatoric strain and the cyclic loading repetition is tudied. The correlation between shear viscosity and resilient modulus is discussed as well. This is the first attempt to find any correlation between soil viscosity and elasticity through laboratory investigations. In contrast，in traditional numerical computations (e.g.，the finite element method)，the extra viscous term added to the governing equation is assumed to be related to system mass and rigidity.