The invention discloses a finite difference frequency domain algorithm-based power calculation method for a power transmission line. In a conventional method, a "path" method is often adopted for line energy transport analysis, active power loss and reactive power loss analysis and natural power calculation, and in a complex cable running environment, a "path" mode is difficultly used for calculating power of a cable. The power calculation method comprises the steps of firstly, deducing a Poynting vector to represent a numerical expression of electromagnetic energy; secondly, calculating an electromagnetic field of a space along the line and energy flux density distribution of the power transmission line based on a finite difference frequency domain method of space stepping to obtain a three-dimensional transmission power representation model of the power transmission line; and calculating power transmission and loss situations based on the three-dimensional transmission power representation model of a finite difference frequency domain algorithm. According to the power calculation method, the power transmission and loss situations can be accurately calculated.