Based on the characteristics of the bottom mud of the Liangxi River, the technology of online dewatering for dredged mud was designed and tested. Through settling column tests and type selection tests for chemicals by computer operation, the turbidity of the supernatant and the volume of the settling column, and the solid content of mud cakes and the discharging time were compared respectively. The chemical selection and dosage were thus analyzed and determined. Through type selection tests for the air permeability of the filter cloth, the solid content of mud cakes and the permeability time under different air permeability were compared. Accordingly, the air permeability of filter cloth was determined. Finally, by applying the data from the above tests on on-site operation of the plate and frame machine, the eigenvalue of the final displacement corresponding to the optimal feeding and back pressure time was determined and used as a reference for pump termination. According to the results of engineering practice, the online dewatering process for dredged mud can meet the design requirements of mud loss rate not less than 50%, mud cake solid content not less than 60%, and mud cake thickness not less than 25 mm, indicating feasibility of the technics.