In order to quickly identify flow transferring in real time, line tripping distribution factors (LTDF) based on DC power flow and compensation principle is deduced to estimate the distribution of multi-line tripping power flow quickly, then combining with the improved severity function, the concepts and solving process of key branch set and overload risk indicator used to estimate impact of flow transferring are introduced in detail, which can apply to single line tripping and multi-line tripping equally and reduce analysis range of subsequent load shedding control strategies. The example analyses of 10-generator 39-bus New England System show that the method can estimate tripping power flow accurately, identify the branches severely affected by flow transferring quickly and efficiently, overcome the deficiency of using distribution factor to evaluate flow transferring degree and miss-selection of important branch, and has the rationality, feasibility and rapidity.