AbstractSludge rheology is an important parameter for pumping, pipeline transportation, and dewatering. This study was designed to investigate the effect of adding iron powder (IP) to thermal hydrolysis pretreatment (THP) sludge and raw sludge on the rheological properties and dewatering performance before and after anaerobic digestion (AD). The rheological data of sludge were also fitted. The results showed that the apparent viscosity of sludge from the combination of IP and THP decreased after AD. The opposite phenomenon can be observed in digested sludge with the addition of IP alone. The Herschel-Bulkley (H-B) model was most appropriate for describing the flow behavior of digested sludge from the combined process, showing the decrease of yield stress (τ0), consistency coefficient (k), and the increase of flow behavior index. The addition of IP can further improve the thixotropy of THP sludge after AD. However, the higher dosage of IP did not show an obvious advantage. The dewaterability of THP sludge with the dosages of IP (30 and 60 mg/g volatile solids) after AD increased by 17% and 19% compared with raw sludge. Notably, the change of dewaterability and soluble chemical oxygen demand were all linear with the thixotropy of digested sludge, which demonstrated that rheological indicators could be a new method to evaluate the changes of physicochemical properties.