Abstract: This article aims to evaluate the electromagnetic interference (EMI) environment in the shell of a system in package (SIP) while irradiated by a pulsed Xray fluecne. First, the yields and energy spectra of photoelectrons induced by Xray were calculated using Monte Carlo numerical simulations. These results were used to simulate the selfconsistent movement of photoelectrons emitting from the Kovar alloy shell and the dielectrical material of printed circuit board (PCB). The electromagnetic pulse (EMP) generated by these photoelectrons were calculated using finite difference timedomain (FDTD) method and particleincell (PIC) method. An analysis of the EMI environment in the shell of SIP was performed by numerical results. It is shown that the EMI environment adjacent to the emission surfaces turns more severe because of the movement of photoelectrons. And most of the radiation energy is located at lowfrequency parts, which depends on the characteristic time of Xray. Also, reducing the area of surfaces illuminated by Xray and the height of SIP can lessen the threat from EMI.