Abstract: The canned motor reactor coolant pumps are used in the Unit 1and Unit 2 in Sanmen Nuclear Power Plant. The lower C seal has to be cut apart by the special way and tool during the pump disassembly. According to the structure of the pump, the functional requirements of the cutting scheme are defined and a cutting toole is designed and developed. The method of finite element analysis (FEA) is used to verify and optimize the structure strength and cooling effect of the tool, to ensure the cutting  accuracy and service life. This cutting scheme and the tool are applicable in the narrow and deep space, and the fixed distance cutting can be conducted with high efficiency, high reliability, and without foreign material and air radiation pollution. The scheme and the tool can be used in the nuclear main pumps maintenance in other nuclear power plants, and has certain industrial value.