Abstract: A dredged silt from irrigation ditches in the Jianghan Plain was treated using a rice-straw-ash-based green cementitious material (denoted as RW), which was prepared by combining modified supersulfated cement (W) with rice straw ash (RSA). The optimal mass ratio of RSA to W was determined through 28-day compressive strength tests. The effects of RW dosage on the compressive strength, water absorption, softening coefficient, pH, and frost resistance of the stabilized silt were investigated. The microstructure and solidification mechanisms were analyzed by X ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the optimal mass ratio of RSA to W is 1∶2. At this ratio, the 28-day strength of specimens with different dosages all exceeds 0.50 MPa. Both the single addition of W and the composite RW can effectively stabilize the sludge. Among them, specimens with only W exhibit higher early strength, whereas a more significant strength increase is observed in RW specimens at later stages. When the dosage is 15.0%, the 90-day strengths of W and RW specimens are close, reaching 4.32 MPa and 4.18 MPa, respectively, which is primarily attributed to the synergistic effect between cement hydration and the pozzolanic reaction of straw ash. The RW specimen with 15.0% dosage demonstrates excellent water resistance and frost resistance, with a water absorption rate as low as 17.32% and a softening coefficient of 0.89. After 10 freeze-thaw cycles, the lowest strength loss and mass loss rates are recorded, which are 7.27% and 4.71%, respectively. The pH of the system gradually increases with higher RW dosage and longer curing age, which promotes the formation of hydration products and pozzolanic reaction products (such as C—S—H and AFt), thereby enhancing the structural stability of the solidified body. This study can provide a theoretical reference for the application of green cementitious materials in sludge stabilization and slope protection in hydraulic engineering.