When the X-ray irradiates metal cavities, a large number of photoelectrons will be emitted into the internal space, resulting a strong System Generated Electromagnetic Pulse(SGEMP). The X-ray environment within laser Inertial Confinement Fusion(ICF) facilities is very complicated, so diagnostics working in target chamber area will face severe SGEMP interference even with good electromagnetic shielding. In light of the X-ray environment inside target chamber of Shenguang-III (SG-III) laser facility, a numerical simulation of the EM field inside a two-dimensional cylindrical cavity model is completed with the time-biased Finite-Difference Time-Domain(FDTD) and Particle-In-Cell(PIC) methods. As for the oscillation of electromagnetic field, based on the classic PIC method, the time-biased arithmetic is utilized to filter the waveform. Therefore, the influence of high-frequency error on the calculation result is removed, and the simulation would be more accurate. Finally, the SGEMP interference in the SG-III laser facility is estimated to be about 1.5 MV/m.