Airworthiness regulations CCAR-25 specify the requirements that aircraft should meet when exposed to high-intensity radiation fields (HIRF). Aiming at the problem of excessive radiation in the electrical and electronic equipment compartment of civil aircraft in the HIRF environment, an optimal design suggestion to improve the HIRF shield capability of the E / E compartment door is proposed. In the study of the HIRF shield capability of the E / E compartment door of civil aircraft, the fuselage with the E / E compartment door model was simplified to a model of a complex metal cavity with an annular slit. Referring to the study of the HIRF shielding capability of the open-hole rectangular cavity, an optimization direction was concluded to improve the HIRF shield capability of E / E compartment door. Three structural schemes were designed, and then low-level swept field (LLSF) tests were carried out to find out if the optimization of the E / E compartment door is effective for improving HIRF shield capability. The test results show that the proposed suggestions are effective for improving the HIRF shield capability.