Abstract: In order to effectively prevent cascading tripping of transmission lines, the calculation method of line tripping distribution factors (LTDF) is deduced to rapidly estimate the power flow distribution after system multi-line tripping based on DC power flow and compensation principle. By combing with improved severity function, the concepts of key branch set and overload severity indicator are given to evaluate impact degree of power flow transferring, and their solving processes are also described, which is also suitable to such cases of single line tripping and multi-line tripping equally. On this basis, load shedding optimization model and load shedding strategy based on the generalized sensitivity and key branch set are studied. Taking a 10-generator 39-bus New England System as example, the results show that proposed method can effectively forecast power flow after system multi-line tripping, quickly eliminate the overload with minimum adjustment quantity, and ensure no overload of other branches on the basis of accurate identification of flow transferring.