The invention discloses a measuring method for online monitoring of corona loss of a power transmission line. The method comprises the following steps of: wrapping a layer of thin metallized film with a certain length on the surface of a power transmission lead, wherein a conductive metal layer is arranged on one outward surface of the wrapped film and an insulated material layer is arranged on one surface of the wrapped film close to the power transmission lead; connecting a sampling resistor between the metal film layer and a power transmission divided conductor in parallel; converting the flowing leakage current into a voltage signal by the sampling resistor; transmitting the voltage signal to a computer on the ground through a wireless mode or an optical fiber; converting the voltage of the power transmission lead is converted through a voltage transformer or a voltage divider and transmitting the converted voltage to the computer; and calculating corona current, loss and other data by computer software. The measuring method disclosed by the invention is suitable for a non-load line and a load line or an alternating current line and a direct current line.