Probability insecurity index is the core of probabilistic model of power system security assessment based on DSR. Through the numerical simulation experiment, to the same transmission line, we discover the change of DSR of different fault position has a certain regularity. According to that, the paper presents a rapid calculation to calculate insecurity probabilistic index based on method of angle even rotation. The method only needs the accurate DSR of the two end points, we can calculate the DSR of other locations on the same transmission line rapidly, and then we can get insecurity probabilistic index. Test on WSCC4-11 system shows the calculation speed increases significantly, the biggest error is about 1.96%, that can meet the needs of the engineering calculation.