The invention discloses a power transmission line broken line locking method based on a negative sequence network; the method comprises the following steps: firstly collecting the power transmission line one side three phase currents and three phase voltages, and obtaining the opposite side three phase voltages through a signal channel; calculating the one side negative sequence current and negative sequence voltage according to obtained information, and calculating the opposite side negative sequence voltage; respectively calculating the negative sequence voltage difference value between two sides of the power transmission line and the negative sequence voltage drop reckoning value according to said data when the power transmission line is assumed to have no broken line fault; comparing the line two-side negative sequence voltage difference value and the negative sequence voltage drop reckoning value, and determining whether the power transmission line has a broken line or not. The method is not affected by multi-loop mutual inductances, thus improving the determination result accuracy, and enabling relay protection to selectively cut off the broken line fault in a heavy current grounding system.