The invention relates to a nuclear power plant large shielded main pump lower portion C-ring cutting device, and belongs to the technical field of maintenance of a nuclear power plant large shielded main pump. The device comprises a wall mounting assembly and a cutting assembly; the wall mounting assembly comprises a pressing plate and a wall hanging body; the pressing plate is fixed to the end surface of a lower end cover of the main pump, a gap is reserved between the pressing plate and the lower end cover of the main pump, and therefore a tongue portion of the wall hanging body can slide inthe gap; the cutting assembly includes an angle grinder head, a transmission mechanism and a cutting mechanism; the lower portion of the wall hanging body is connected to the angle grinder head, andthe angle grinder head is connected to the cutting mechanism extending into a space where a C ring of the main pump is located through the transmission mechanism. The nuclear power plant large shielded main pump lower portion C-ring cutting device is suitable for a narrow and deep cutting environment, can conduct online cutting, is high in efficiency, controllable in precision, smooth in cutting slit and controllable in foreign matter, and does not cause air radiation pollution.