We present theoretical research on the intensive electromagnetic pulse (EMP) inside a target chamber. Three types of EMP environment, including one irradiated by escaped hot electrons, one involving cavity-system-generated EMP (SGEMP), and one involving cable SGEMP, are considered based on different physical mechanisms. Further, we introduce the physical model and the numerical simulation method. The simulation is conducted based on the finite-difference time-domain method, the particle-in-cell method, and the Monte Carlo algorithm. The theoretical results are in good agreement with the experimental results. This study provides technical support for further studying on the electromagnetic phenomena in the laser-target processes and improving the electromagnetic compatibility of the devices.