The early-time high-altitude electromagnetic pulse is an important component of the electromagnetic effect of high-altitude nuclear burst. The relative simulation research work at home and abroad are studied and compared. One of the classical methods EXEMP is introduced in this paper in detail. The numerical result is compared with the IEC reference value of IEC 61000-2-9. The time domain waveform and spatiotemporal distribution of HEMP are studied using numerical result. The influence of burst parameter including energy and height are studied, the uncertainty is quantified by polynomial chaos method and global sensitivity indices.