A main shaft structure of a tool machine includes a base, a worktable, a motor fixing seat, a cutter seat, and a cutter wheel unit. The worktable is mounted on the base. The motor fixing seat is connected to one side of the base, and another side is connected with the cutter seat. Two ends of the cutter seat respectively have bearing recesses and first positioning portions close to the bearing recesses. The cutter wheel unit is coupled to the cutter seat. Two ends of the cutter wheel unit are provided with bearing seats which are placed into the bearing recesses. The bearing seats have second positioning portions to connect with the first positioning portions, so that the cutter wheel unit can be detached from the cutter seat quickly.
