In order to determine the vulnerability of electronics system to Electro Magnetic Pulse(EMP), and to design the electronic systems which can withstand the threat of EMP，the EMP characteristics and the energy for EMP coupling into components and electronic system are studied, as well as the influence caused by degraded tactical performance of electronic systems. The design process of small electromagnetic field sensor in the closed cavity for resisting strong electromagnetic interference is introduced. Detailed analysis through theoretical calculation on the equivalent circuit of the sensor, matching network and measurement method is performed. The development process and actual applications are also introduced. Internal Electro Magnetic Pulse(IEMP) environment parameters are obtained in the small cavity body. This work provides a reference for the anti-electromagnetic interference reinforcement design of electronic systems.