Abstract: Many scholars have adopted the transient pulse method to measure rock permeability and used the time series of differential pressure to calculate seepage velocity series. They have found that the seepage velocity doesn’t have linear relation with the differential pressure gradient, so they have thought that there exists non-Darcy flow in post-peak rock. To understand the problem, an analytic solution to the unsteady seepage problem in the transient pulse method is solved, and it is found that the seepage velocity determined by the transient pulse method is not linear with the differential pressure even if the Darcy’s law is suitable. Moreover, by analyzing the relationship between permeability and lateral displacement of post-peak rock and by calculating some Reynolds numbers of flow in post-peak rock, it is found that the Darcy’s law is still suitable for them. So, it needs more researches on whether there is non-Darcy flow in the researches reported by far. But, non-Darcy flow will surely occur when the hydraulic gradient and the fracture aperture are amplified.