The invention discloses an optimization method for current variable quantity calculation of travelling-wave protection of an extra-high-voltage direct-current power transmission line. At an inverter side, a line current IdLH is used as an input and a time delta T is used as a differential constant; a direct-current component that is not caused by a fault is changed into zero by differential processing and an alternating-current component caused by a fault is processed by differential processing to obtain a variable quantity at a sampling interval delta T. When a line voltage change rate meets a requirement, the result after differential processing is processed by integration processing to be represented by a changing value after reduction of the direct-current component from the line current within effective integral time; and the changing value is compared with a threshold value. With the optimization method, travelling-wave protection maloperation caused by defects of the calculation processing method can be avoided.