In order to obtain an appropriate blocking algorithm of double terminal travelling wave fault location, so that it can block the subsequent catadioptric wave and open the fault location algorithm as soon as possible, this paper analyzes the refraction and reflection of the traveling wave on the line. It analyzes all kinds of the current reflection and refraction coefficients at surge impedance discontinuous point, and calculates the time-amplitude relationship between the initial wave front and the subsequent catadioptric wave front in the internal fault, forward external fault and reverse external fault. It provides an appropriate blocking algorithm:block the fault location in 5 ms after fault occurring, block the wave front that greater than 60% initial wave front in 5~40 ms, and open fault location after 40 ms. At last, the EMTP simulation proves the effectiveness of the blocking algorithm. This work is supported by Key Technology Project of China Southern Power Grid “Development of High Precision Digital Simulation Experiment Research Platform for Traveling Wave Fault Location” (No. KYKJXM00000006).