The invention discloses a method and method for judging a weak line of a regional power grid at a stable transient power angle. Through the initial power flow of the power grid, line fragile factor (LFF) matrixes of different lines of the power grid in a certain region can be calculated via simple information acquisition and matrix operation after different types of faults occur; and the weakest line and the most serious fault of the system at the stable transient power angle can be directly judged through the LFF matrixes. The method is simple and direct, the influence on the weak link of thesystem and other lines on the weak link is clear at a glance, the simulation time can be greatly saved, and the method can be used for various types of analysis of a power system.