High-concentration tailings slurries exhibit complex thixotropy that is significantly affected by particle size of tailings though the relevant studies are not abundant. To reveal the influence of particle size on thixotropy of tailings pastes, samples of various mean particle sizes prepared from the same tailings were applied in the constant shear rate experiments. Results show that the tailings pastes of interest display significant thixotropy that leads to the shear-thinning phenomenon under steady shear rates. In the steady state, Bingham yield stress, equilibrium yield stress, and Bingham viscosity appear to correlate linearly to the reciprocal of square of mean particle size. As for the transient state, the corresponding fit parameters show strong linear dependence on the mean particle size. The proposed equilibrium and thixotropic models are valid. The forecasting models for equilibrium and transient shear stress are established based on data fit, which attributes to the quantitative characterization of steady-state and transient rheology for thixotropic tailings pastes under the effect of particle size.