The invention provides a compensation method for reactive loss of a power transmission line. The method comprises the steps that the electromagnetic field intensity of the cross section of any one end of the power transmission line is calculated; a space stepping frequency domain finite difference algorithm is used for segmenting the power transmission line into a plurality of cross sections, and the electromagnetic field intensity of the cross section of any one end is used as a reference parameter for deriving the electromagnetic field intensity of other cross sections of the power transmission line; according to the electromagnetic field intensity of all the cross sections of the power transmission line, and an electric field energy change rate and a magnetic field energy change rate in unit volume are calculated and integrated for obtaining electric field energy loss and magnetic field energy loss; the sum of the electric field energy loss and the magnetic field energy loss serves as the reactive loss of the power transmission line for adjusting output reactive compensation power. According to the method, the movement of the power transmission line serves as an energy loss calculation factor for calculating the change, caused by the sag of the line, of the reactive power, the final calculation result can be more accurate, and therefore the accuracy for reactive compensation of the power transmission line can be improved.