Power loads are sensitive to voltage sags and short interruptions (the following is called flash broken). The demand for reliability of power supply is higher and higher. According to the flash broken of power grid, this paper analyzes the causes of this phenomenon. Distance protection as the main protection of transmission line, is inevitably affected by flash broken. The direction of the phase comparator mode impedance relay is used to build Matlab/Simulink distance protection model, and the correctness of the three-stage distance protection is verified. The operation principle of the individual elements and the effect of each component in the protection distance are described, especially for starting element. By setting different types and different lengths of line fault, the different effects of flash broken on distance protection is simulated.