The invention relates to the technical field of a tool for mechanical processing, in particular to a tool for processing a long and thin shaft sleeve for an axially split pump, which is characterized by comprising a boring cutter rod and a lathe carrier assembly, wherein the boring cuter rod is of a long and thin shaft rod, the central shafts of the two ends of the boring cutter rod are respectively provided with a tip positioning hole for jacking and positioning a tip, the boring cutter rod is radially provided with more than one floating cutter heat installing hole for installing floating cutter heads with different diameters, the floating cutter heads are arranged in the installing holes, one end of the boring cutter rod is provided with the lathe carrier assembly, the lathe carrier assembly comprises a positioning hoop, a fastening bolt and a torque rod, the positioning hoop is sleeved on the boring cutter rod and is radially provided with a positioning screw hole, the upper end of the fastening bolt is fixed with the torque rod in a crossed way, the lower end of the fastening bolt passes through the positioning screw hole in the fastening bolt to be contacted with the boring cutter rod, so that the boring cutter rod is fixed with the positioning hoop, and further, the lathe carrier assembly can transmit torque. Due to the adoption of the structure, the tool has the advantages that the structure is novel, inner holes at the two ends of the shaft sleeve can be processed at one-time clamping, the coaxiality and surface roughness of the inner holes can be ensured, etc.