Fluid flow in fractures is one of the important factors affecting the stability of rock masses. The fluid in the fracture not only produce pressure but also weaken the fracture strength. therefore, engineering accidents cause frequently including slope instability and tunnel collapse. In order to explore the action mechanism of seepage on the block stability of fractured rock masses, the study proposes a stability analysis method considering both seepage force and fracture strength water weakening, based on the Key Block Theory (KBT). In this method, the fluid pressures in fractures are simulated by Unified Pipe-network Method (UPM), and the relationship between fracture strength's weakening and fluid pressures is determined by an empirical formula from. This method is adopted to study the influencing of seepage on the stability of a fractured rock slope. The research results show that the action modes of seepage pressure and fracture water weakening on the slope stability of fractured rock masses is different. The seepage force change both the anti-sliding force and sliding force of the block, Therefore, the influence of seepage force on the stability of the block is complicated, including changes of the block stability states, sliding surface, and the instability pattern. fracture water weakening only reduces the anti-sliding force of blocks, and then reduces the stability of block, but does not change the instability pattern of unstable block. Moreover, there is usually a critical pressure which is the most dangerous pressure for rock slope engineering. This method provides theoretical support for the stability study of fractured rock masses.