Mercuric iodide (HgI2), as a semiconductor detecting material directly absorbing X-ray, its inherent spatial resolution has been investigated for the medical diagnosis X-ray energy. The Monte-Carlo simulation is performed with an infinitesimal pencil-beam of X-ray incident on the HgI2 slab to determine its modulation transfer function(MTF). The simulation is based on the user code DOSRZnrc of the latest version of the EGSnrc, and these effects on HgI2 spatial resolution by the reabsorptions of fluorescence and scattered photons, expanded ranges of fast photoelectrons and off-axis incident angle, are considered and the simulated results accord with the literature analytic results well. The simulation results show that the ranges of primary fast photoelectrons and off-axis incident angle can affect the spatial resolution of the material greatly and the spatial resolution is sensitive to the incident X-ray energy since the modulation transfer function (MTF) decreases greatly with the increasing of X-ray energy, which however depends hardly on the thickness of the film. Compared with amorphous Se, HgI2 has a higher inherent spatial resolution, especially for above 50 keV X-ray energy. When fMTF=0.5, for X-ray photon energy of 20 keV, 50 keV and 100 keV, the inherent spatial resolutions of HgI2 and amorphous Se are 390 lp/mm, 170 lp/mm,52 lp/mm and 390 lp/mm, 80 lp/mm, 22 lp/mm, respectively.