Aiming at the stress concentration problem existing in the front part of the highvoltage cable joint protective device, this paper presents a method to balance the stress distribution by optimizing the deflection angle of the folding line and the flange size. Finite element method of temperature field, flow field and displacement field coupling is adopted to calculate the stress value of the front part of joint protective device inner cavity on the 220 kV metal cable under different structure. Thus, the stress distribution of protective inner cavity is obtained. Through simulation analysis, combined with the practicability and economy of the protective device, the final chosen deflection angle α of the 220 kV metal cable joint explosion proof device folding line on the front part is 60°. The optimal structure of the flanges added at the fronttoshell connections is determined as 50 mm in height and 20 mm in width, which can reduce the stress distortion coefficient of the inner wall of the protective device to 1.48. The reliability of the design method is verified by the explosion stress test of the protective device of 220 kV highvoltage cable joint with broken line terminal. By comparing the polygonal structure to the curved structure, it is considered that under the same technical conditions, the polygonal structure has better economic benefits. The paper provides a reliable theoretical calculation method for the design of the protective device.