The invention relates to a method for measuring overvoltage of an electric power system power transmission line and belongs to the technical field of overvoltage measuring. The method includes: performing overvoltage simulation on an electric power system to calculate the theoretical multiple of power transmission line overvoltage and power frequency voltage, setting the multiple the triggering voltage of the power transmission line overvoltage and power transmission line power frequency voltage, allowing an optical electric field sensor to sense voltage in the power transmission line, processing to obtain power frequency voltage U1, and setting the triggering voltage of the power transmission line overvoltage to be k1*U1; detecting the voltage of the electric power system power transmission line in real time to obtain output voltage U2, judging U2 to obtain voltage magnitude U3, and acquiring the power transmission line overvoltage multiple k2 according to k2=U3/U1. The method has the advantages that the method is suitable for long-term overvoltage monitoring and also applicable to overvoltage measuring during tests and debugging; compared with other overvoltage measuring means, the method does not need calibration before tests, and mounting convenience and operation simpleness are achieved.