By the aftershock frequency estimation method based on the calculation of coseismic static Coulomb stress changes and rate-and state-dependent fault constitutive law，we calculate the frequency of “direct”aftershocks of the Wenchuan earthquake related to coseismic static Coulomb stress changes in its aftershock zone and the areas nearby． It shows that the frequency is significantly lower than the truth in the main rupture zone，especially in the southern rupture zone，due to the decrease of stress level on the rupture plane of the main shock resulting from coseismic Coulomb stress change． The study also shows that the effect of the Coulomb stress change on the duration of aftershock activity is associated with the lower limit magnitude． The duration is about 15 －16 months for aftershocks above ML 4. 0，and close to 60 months for aftershocks above ML3. 5． In this period，the ratio of the “direct”aftershocks caused by coseismic Coulomb stress change ranges between 44. 7% to 48. 6%，which suggests that，even in the“effective”period of coseismic Coulomb stress changes，about half of the aftershocks on the main shock rupture plane are independent of coseismic Coulomb stress changes． It is pointed out that those aftershocks may be related to the afterslip or the viscoelastic relaxation，which are time dependent cases．