Abstract: The failure mechanism formed under the action of circular medium fluid around the wellbore of brittle shale has been a hot issue in recent years. The water activity difference between the drilling fluid and shale will directly affect the formation mechanism brittle shale fracture. The model of activity-inflation-hydration is set up firstly through revising generalized Usher model. For this model, it states that the wellbore is in non-osmotic hydration stage within the critical expansion ratio, and thus puts forward the concept of activity window according to the drilling fluid activity range, and this is verified by indoor test. Results show that the different activity of drilling fluid brings difference in rock strength in macro and time sensitive effect for internal structure of rock in micro. This mechanism can provide a more accurate range design for drilling fluid activity and strong guarantee for wellbore stability.