In order to meet the need of large strain range measurement on the key structures of ship, bridge, aircraft, and so on, we design a multi-loop desensitized fiber Bragg grating (FBG) strain sensor based on low temperature plasma. Finite element analysis of a tricyclic desensitized strain gauge substrate with a length of 30 mm is carried out with a displacement of 0.083 mm on each of the left and right end faces. After calculation, the strain in the gate region is about 1700 με and the desensitization coefficient of the structure is 2.91. Two contrast groups are adopted to verify the sensing characteristics within +5000 με; low temperature plasma is used to treat the gate region surface for one group sensor; while the other is set as default. The results show that the more times the plasma discharge treates the gate region surface, the larger the fiber optical power becomes. The average measurement error of the system is about 50 με, and the full-scale accuracy is less than 0.5%, which solves the problems of instability and poor linearity of the system calibration results caused by the pollution of the gate end face during the packaging process.