The invention relates to a determination method of a power transmission line actual lightning protection angle. The method consists of: first making use of an existing two-dimensional power transmission line corridor distribution map and a sea-level elevation map to conduct scanning and projection overlay analysis so as to obtain a three-dimensional power transmission line corridor distribution map, and determining the three-dimensional geographic coordinates all over a power transmission line, then making a vertical line to an arbitrary span section of the power transmission line, and determining the properties of an only plane according to two intersection lines in the space, thus obtaining an error plane of the lightning protection angle within the span section and its positive direction vector, then subjecting an included angle of the error plane positive direction vector and a rightarrow (Z) axis of a Cartesian coordinate system to inverse cosine calculation to obtain an error angle under the power transmission line error plane, and finally according to the relative position of three phase leads and a tower, carrying out error angle correction, thus obtaining the actual lightning protection angle of each phase lead. The method provided in the invention realizes remote, accurate, and rapid determination of the power transmission line actual lightning protection angle, and has very good operability and practicality.