Based on the large shaking table test of saturated sand on the free field, the dynamic shear stress–strain response of the foundation soil model was obtained by using the linear interpolation method according to the data from accelerometer arrays. At the same time, the concepts of apparent viscosity and zero shear viscosity of non-Newtonian fluid based on the assumption of hydrodynamic theory were introduced to study the characteristics and behaviors of shear thinning after site liquefaction. The characteristics of solid-liquid phase transition of saturated sand under seismic load were studied. Results showed that when the saturated sand was stimulated by a large earthquake, excess pore water pressure accumulated rapidly, and sand liquefaction occurred as the pore water pressure ratio of the upper part of the saturated soil ran up to 1. According to the variation of dynamic shear stress–strain curve, the dynamic shear modulus of saturated sand decreased significantly after liquefaction, indicating that the soil softened gradually. The variation trend of shear stress-shear strain rate of saturated sand derived from the data of dynamic shear stress and dynamic shear strain was similar to the rheological curve of non-Newtonian fluid. The apparent viscosity of the liquefied soil in the upper part of saturated sand layer decreased significantly. After liquefaction, the sand showed the pseudoplastic fluid characteristics of "shear thinning".