The invention belongs to the technical field of power system cascading failure prevention and defense, and particularly relates to an AC-DC power grid cascading failure key line identification method. The method comprises: calculating a weight value of each AC line in an AC-DC power grid; calculating a power grid transient stability process after a DC line or an AC line is tripped due to a failure; calculating transmission capacity margin variation of a surrounding line caused by active power of the failure line; calculating flow transferring correlation of the failure line on the surrounding line; calculating a line ratio through a ratio of the flow transferring correlation of the failure line on the surrounding line to a mean value of the sum of flow transferring correlations of all lines of the power grid on the surrounding line; and obtaining all line ratios, wherein the line with highest ratio is a line with highest key degree. The method considers both flow transferring in the process of occurring the cascading failure and DC system operation characteristics, can perform quantitative analysis on key degrees of all the lines, and is of great significance on cascading failure prevention and control.