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We investigated the magnitude–frequency relationship and decay pattern of an aftershock sequence using data from the 2008 Wenchuan earthquake. We analyzed the spatial variations in aftershock activity parameters <i>b</i> and <i>p</i>. The calculated <i>b</i>-value of the aftershock sequence is 0.89 ± 0.02, which is relatively small, probably owing to the absence of small earthquakes in the aftershock catalog. The <i>p</i>-value, indicating the decay rate of aftershock activity, is 1.05 ± 0.02, which is normal. The decay pattern of the Wenchuan aftershock sequence agrees well with the modified Omori law. The <i>b</i>-value of the aftershock sequence mainly spatially varies between 0.6 and 1.2, and the <i>p</i>-value varies between 0.6 and 1.8. Although the physical significance of the spatial variations in <i>b-</i> and <i>p</i>-values has not been clearly defined, in this study, the physical significance of the <i>b</i>-value is mainly related to changes in stress, <i>P</i>-wave velocity, and the density of media in the earthquake area, and that in the <i>p</i>-value is associated with the fault slip amount during the mainshock; the <i>b-</i> and <i>p</i>-values show a strong linear correlation. After the mainshock, stress decreased and increased in areas with large and small <i>b</i>-values, respectively; the regions with large and small <i>b</i>-values were associated with low and high <i>P</i>-wave velocities, respectively. The subsurface media experienced relatively high and low apparent velocities in areas with small and large <i>b</i>-values, respectively. The amount of fault slip was small and large in regions with small and large <i>p</i>-values, respectively, exhibiting a linear correlation between the fault slip amount and <i>p</i>-value. The results indicate that the spatial variations in the <i>b-</i> and <i>p</i>-values were related to the physical properties of the media in the earthquake area and focal earthquake mechanism.