The invention relates to the technical field of a sludge dewatering device, and discloses an automatic compound conditioning device and a conditioning method for sludge dewatering. The device comprises a conditioning tank, a plurality of liquid conditioner feeding systems, a plurality of solid conditioner feeding systems, a control system and a stirring system, wherein the conditioning tank can contain sludge which needs to be conditioned; the control system can respectively and correspondingly control the liquid conditioner feeding systems and the solid conditioner feeding systems to complexly feed liquid conditioner and solid conditioner to the conditioning tank; and the stirring system stirs sludge in the conditioning tank and respectively stirs the liquid conditioner in the liquid conditioner feeding systems. In the whole sludge conditioning process, the control system comprehensively controls the liquid conditioner feeding system completely according to the setting of a preset program, and various different liquid conditioners and solid liquid conditioners can be complexly fed simultaneously or in sequence according to the practical requirement. The automatic compound conditioning device has the advantages of high automation degree and high working efficiency and is convenient to operate, and high cost can be lowered.