The invention provides a sensor used for part inner surface quenching. The sensor comprises a sensing ring made from a steel structure. The sensing ring comprises a plurality of left hollow semi-rings and a plurality of right hollow semi-rings. The left semi-rings and the right semi-rings are alternately overlapped in the up-down direction without intervals. The projection of adjacent left semi-ring and the right semi-ring in the vertical direction is in a circle shape. One end of one left semi-ring communicates with one end of the adjacent right semi-ring in the vertical direction, and an insulating piece is arranged between the other end of the left semi-ring and the other end of the adjacent right semi-ring. Inner cavities of all the left semi-rings and inner cavities of all the right semi-rings communicate into a spiral passageway. The passageway communicates with a water cooling device. A quenching liquid spraying pipe is arranged in the sensing ring in a penetrating mode. The left semi-rings and the right semi-rings of the sensing ring are arranged without intervals in the vertical direction, the whole part inner surface can be heated, the temperature is even, the effect that a hardened layer after quenching is even is ensured, the production efficiency is improved, and the part product quality is ensured.