Abstract: Paste backfilling technology, by which tailings paste is transported into underground stopes, does not only relieve the environmental pressure caused by increasing volumes of tailings ponds, but also averts the potential dangers of underground goaf. However, because of the complex characteristics of some mine tailings, it is challenging to adopt the paste backfilling technology for tailings disposal, as the pipeline transportation of highly muddy pastes is difficult because of the poor flowability caused by high yield stress. To improve the flowability of tailings pastes, a rheological experimental setup have been designed to investigate the mechanism of how pumping agents affect the rheological properties of a highly muddy paste. The results show that a significant linear relationship exists between the tailings paste concentration and corresponding yield stress, with different pumping agent dosages. Further analysis reveals a significant exponential relationship between pumping agent dosage and slope (or intercept) of the linear function. Finally, the prediction function for the yield stress is proposed, considering the pumping agent dosage and tailings paste concentration. This can characterize how the pumping agent influences the rheological properties of a highly muddy paste and enable precise forecasting of the pumping agent dosage and management of slurry flowability. Based on the prediction model, the effect mechanism of pumping agent on a highly muddy tailings paste rheology is proposed. Based on the analysis of the images observed from an environmental scanning electron microscope (ESEM), it is found that the presence of pumping agent causes the destruction of flocs in tailings pastes. Meanwhile, the optimal dosage of pumping agent suggested by experiments is about 1%. Furthermore, the pumping agent accounts for floc size shrinkage and consequently yield stress reduction. Moreover, increasing the distance between broken flocs weakens their interactions and slows down the flocs destruction rate. The observations agree with the theoretical analysis results.