Abstract: High-concentration tailings pastes are commonly encountered in the surface deposition and underground cemented paste backfill. It is necessary to forecast the pressure loss along the pipe and the flow behavior in the mined-out area when tailings pastes are transported in a pipeline or stored up in goaf. Traditionally, thixotropy has often been overlooked while designing tailings paste transport and disposal systems due to the complexity of thixotropy characterization. However, thixotropy plays an essential role in the rheology of tailings pastes. To achieve the accurate representation of thixotropy, a suitable constitutive model has to be selected in advance. Unfortunately, there is no current available thixotropic model for tailings pastes up to date. Moreover, tailings pastes exhibit complex thixotropy that is significantly affected by the particle size of tailings, though the relevant studies are not abundant. It is important to analyze quantitively the relationship between particle size and thixotropic parameters based on an appropriate constitutive model. In this work, the constitutive thixotropic model for tailings pastes proposed by Zhang et al. was adopted. To reveal the influence of particle size on the thixotropy of tailings pastes, samples of various mean particle sizes prepared from the same tailings were applied to constant shear rate experiments. Results show that the target tailings pastes display significant thixotropy, leading to shear thinning under steady shear rates. In the steady state, the static yield stress, dynamic yield stress, and Bingham viscosity appear to correlate linearly to the reciprocal of the square of the mean particle size. As for the transient state, the corresponding fit parameters show a 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 is attributed to the quantitative characterization of steady-state and transient rheology for thixotropic tailings pastes under the effect of particle size.