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 b and p. The calculated b-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 p-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 b-value of the aftershock sequence mainly spatially varies between 0.6 and 1.2, and the p-value varies between 0.6 and 1.8. Although the physical significance of the spatial variations in b- and p-values has not been clearly defined, in this study, the physical significance of the b-value is mainly related to changes in stress, P-wave velocity, and the density of media in the earthquake area, and that in the p-value is associated with the fault slip amount during the mainshock; the b- and p-values show a strong linear correlation. After the mainshock, stress decreased and increased in areas with large and small b-values, respectively; the regions with large and small b-values were associated with low and high P-wave velocities, respectively. The subsurface media experienced relatively high and low apparent velocities in areas with small and large b-values, respectively. The amount of fault slip was small and large in regions with small and large p-values, respectively, exhibiting a linear correlation between the fault slip amount and p-value. The results indicate that the spatial variations in the b- and p-values were related to the physical properties of the media in the earthquake area and focal earthquake mechanism.