The invention relates to a polishing wheel expansion assembly for a lathe. The polishing wheel expansion assembly comprises a clamping block, a connecting support, a polishing wheel and a driving mechanism. The clamping block is installed on a tool rest of the lathe. The connecting support comprises a rotating support and a pressing support, a rotating rod is arranged on one side of the rotating support, the rotating support is connected with the clamping block in a rotating mode through the rotating rod, the pressing support is fixed to the rotating support and comprises two floating assemblies, each floating assembly comprises a guide rail frame, a sliding block is arranged in each guide rail frame, and one side of each sliding block is elastically connected with the axial end of the corresponding guide rail frame. The two sides of the polishing wheel are connected with the sliding blocks in a rotating mode. The driving mechanism comprises a power source, an adapter block is arranged on a rotating shaft of the polishing wheel, a closed annular gear rack is coaxially arranged on the polishing wheel, a power input shaft is connected to the adapter block in a rotating mode, a gear meshed with the annular gear rack is arranged on the power input shaft, and the power source is connected with the power input shaft through a universal flexible shaft.