The mechanism and characters of diffraction radiation of subwavelength metallic grating structure are analysed and discussed. Using the three dimensinal particleincell simulation, the contour map of electric field of the grating is obtained excited by moving electron bunch. Combined with the Brillouin diagram, it shows that the diffraction radiation in the upper and lower halfspace can also be explained by the formula of SimthPurcell radiation. Then, the influences of the gap width and the thickness of the grating to the diffraciton radiation are also analyzed. Based on the comparison for the asymmetric graing excited by different beam energies, the physical mechanism of the diffraction radiation in the lower halfspace are discussed in further. For asymmetric grating, the diffraciton radiation in the lower halfspace are determined by the velocity of elelctron and the grating period, and by the radiation in the upper halfspace.