Three agricultural biomasses, wheat stalk powder (WSP), corn stalk powder (CSP), and rice husk powder (RHP) were used to enhance sludge dewaterability at different pH values. Capillary suction time (t_CS), filtration resistance (SRF, F_SR), etc. were used to characterize sludge dewatering performance. Sludge particle size and Zeta potential were investigated to reflect the changes of sludge properties. Turbidity, extracellular polymeric substance (EPS), bound water, and sludge cake structure were used to explain the mechanism of synergistic effect. The results showed that the optimum sludge dewatering performance was achieved when the pH value was 3 with a dewatered sludge cake moisture content of 61.5% under 2 MPa pressure. When WSP, CSP, and RHP were dosed separately, F_SR decreased to 7.32×10⁸, 7.41×10⁸, and 8.29×10⁸ s²/g, respectively. Sludge t_CS and F_SR increased remarkably with pH values lower than 2 or ranging from 8 to 11, and 0.75 g/g DS biomass could not improve sludge dewaterability, especially under strong alkali conditions. Strong acidity and alkali made sludge release more turbidity and organic matter which would block sludge cake and filter medium. The mechanism of acid and biomass improving the sludge dewatering performance of sludge is that acid (pH=3) promotes sludge extracellular polymerization (EPS) decomposition, while biomass provides water pore channels for dewatering as skeleton builder.