Based on probabilistic seismic risk analysis result of 8181 grids in Sichuan and adjacent region, we summed up the features of ratios between peak ground accelerations of different risk levels. We focused our research on the statistical and spatial distribution characteristics of the ratio (K2-value) between rare and medium seismic action and made a preliminary analysis on the factors affect K2-value. The result could provide not only a significant basement for determining criteria and methodology for compiling seismic zonation maps against collapse of structures, but also a reference to aseismic design and checking computation for different industries and engineering.