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Abstract Bioleaching, a technologically and economically feasible technology, is considered as the high efficiency method to improve dewaterability in sewage sludge. The objective of this study was to investigate the effect of different sludge concentrations on bioleaching dewaterability and figure out the mechanism of bioleaching affect sludge dewaterability. Variation in pH, oxidation reduction potential (ORP), capillary suction time (CST), specific resistance to filtration (SRF) and different fractionations extracellular polymeric substances (EPS) including slime EPS (S-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) were determined. Different sludge concentrations (5, 10, 15, 20 and 30 g·L−1) were selected to investigate during bioleaching. Results indicated that sludge buffering capacity significantly inhibited bioleaching efficiency as sludge concentrations increase. Optimum enhancements in sludge dewaterability were observed during the 10 g·L−1 sludge concentration treatment, and reached a maximum when pH was 2.11. The variation of different fractionations EPS revealed that the ratio of S-EPS/TB-EPS significantly affected sludge dewaterability. Principal component analysis and Pearson's correlation analysis both provide evidences that the higher TB-EPS and holding a tremendous reduction has positive correction with sludge dewaterability. However, the increase of protein and DNA in S-EPS content has negative correction with sludge dewaterability.