In order to better identify the critical lines in power grid that may lead to cascading failures of power system, a new method based on comprehensive betweenness index and power transmission efficiency is proposed. Grounded on the power flow and distribution of power grid’s tidal current, this method has taken the power transmission margin of the line and post-breakdown transfer of power flow into full consideration, and quantified the role of critical lines in the power grid’s structure and motion, whose physical background is more accord with the practical situation of power system. Meanwhile, to validate whether the critical lines has a dominant role in the transmission of cascading failures, the power transmission efficiency index that has combined network transmission efficiency with directivity of power flow is put forward. The simulation result of IEEE-39 standard test system demonstrates that comprehensive betweenness index has a better performance in identifying critical lines in power grid.