A fiber Bragg grating（ FBG） temperature sensor with copper ring mounted on the middle of cold contracting insulation tube is developed in order to timely detect fault of cable joint. The sensor is applied to monitor temperature of middle joint made by XLPE cable of 3 + 1 core with the rated voltage of 1 kV

cross-section area is 25 mm2. When supply voltage is 5

15 kV within 40 min

temperature of middle joint with burr is 2. 45

4. 31 ℃ higher than temperature of normal middle joints because partial discharge phenomenon is caused by increasing of local field strength. The experimental results indicate that this sensor can effectively monitor the temperature of the middle joints and provide important evidence for the normal work of the middle joints.