The invention discloses a setting calculation method for a protection setting value of a power transmission line zero-sequence overcurrent protection starting element. The setting calculation method settles a technical problem of abnormal starting of the power transmission line zero-sequence overcurrent protection starting element on the condition of overlarge unbalanced current of the power transmission line caused by unreasonable setting value of the existing power transmission line zero-sequence overcurrent protection starting element. According to the setting calculation method, an unbalanced current calculating model of the power transmission line is established according to tower type, lead/ground wire parameter and lead phase sequence arrangement and line length of all power transmission lines which are connected with the bus of a transformer substation, thereby calculating positive-sequence current, negative-sequence current, zero-sequence current and zero-sequence current unbalance factor. After the transformer substation comes into operation, secondary zero-sequence unbalanced current of all power transmission lines are monitored. Finally the protection setting value of the power transmission line zero-sequence overcurrent protection starting element is set according to whether the zero-sequence unbalance factor is higher than 5%.