Abstract: The permeability of fractured rocks is an important hydraulic property for seepage analysis and design of seepage control system in the surrounding rocks. The water pressure test in borehole is an effective technique for characterizing the permeability in the fractured rocks.With the mirror-image method,a Darcy’s law-based analytical model is established and numerically verified to estimate the hydraulic conductivity of the tested rocks. Given that the expression of hydraulic conductivity k is consistent with the Hvorslev equation recommended in the currently-applied codes of borehole water pressure test under certain conditions,the proposed model is hence an extension of the recommended formula. With its simple form and clear physical meaning,the proposed model can be treated as an important basis for revision of the codes of water pressure test in borehole.