The invention relates to the technical field of internal temperature measurement of metal, and discloses a novel inner wall temperature measuring device and method of metal. A measured metal sample in the device comprises a front segment and a rear segment, and the front segment and the rear segment are fixedly connected via loop threads, and are sealed via a planar sealing gasket arranged between the front segment and the rear segment; a temperature measuring groove is formed in the right end surface of the front segment of the measured metal sample, holes are formed in the tube wall of the front segment of the measured metal sample along the axial direction, the holes are lead out along different axial lengths or angles, a fiber temperature sensor positioned in the temperature measuring groove is led out of the holes in the front segment of the measured metal sample along the axial direction, and the fiber temperature sensor is connected with a joint of a fiber sensor by laser welding. According to the device and method, the wall temperature along the thickness direction of the metal can be measured, the wall temperature of the metal can be measured more accurately, and rapid response of metal wall temperature measurement is realized.