Abstract: MCNP was adopted to give the irradiation environment and the electron emission parameter of each internal emission surface under the low energy fluence X ray illuminating perpendicularly to the external head surface of the cylinder and penetrating inside. And the 3D particle-in-cell (PIC) code was combined to calculate the electromagnetic field and particle distribution when different surfaces emit electrons at the same time, and the results were compared with those when only the internal head surface was set as the emission surface. The result indicates that on the account of the plenty of electrons emitted from the side wall and end surface in the cavity, the electric field intensity on the head surface is enlarged significantly over 2 times, while because the electrons are emitted from different emission surfaces, the total current is relatively complex, resulting in the decrease of the magnetic field intensity. Meanwhile, two groups of normal electric field intensity under different energy distributions from MCNP and approximate value by early references separately were compared. The result shows that energy distribution will greatly change the electromagnetic field, and it is still necessary to calculate energy distribution of IEMP by Monte Carlo codes.