The invention discloses a tool for vertical lathe machining of a ring-cone-shaped thin-walled part. The tool for vertical lathe machining of the ring-cone-shaped thin-walled part comprises a tool base plate, a plurality of expanded sliding blocks and a conical block. The tool base plate is in a disk shape, and a through hole is formed in the center of the tool base plate. One end of a mandrel penetrates through the through hole in the center and then is fixed to the tool base plate. One end of each expanded sliding block is used as a clamping face, the outline of the end face of each expanded sliding block is matched with the curved surface of the inner wall surface of a workpiece to be machined, the other end of each expanded sliding block is used as a driving end, and the end face of the driving end of each expanded sliding block is an inclined face. The conical block is connected to the mandrel in a sleeving mode, and the conical surface of the conical block is matched with a conical space. The conical block can be driven to conduct interaction with the conical space, so that the expanded sliding blocks are driven to slide on guide rails in the radial direction to enable the clamping faces of the expanded sliding blocks to abut against the inner wall of the workpiece, and therefore the workpiece is clamped and fixed to be machined. The invention further discloses the application of the tool to vertical lathe machining. The tool for vertical lathe machining of the ring-cone-shaped thin-walled part can clamp and fix the ring-cone-shaped thin-walled part, it is guaranteed that when the workpiece is machined, the gaps between the workpiece and clamps are even, the workpiece will not slide axially, and therefore the machining precision is high. In addition, the tool is convenient to assemble and disassemble.