The invention provides a lightning protection performance calculation method for a power transmission line without a lightning conductor, and the method comprises the steps: carrying out the calculation through a regulation method and a statistical method according to the specific structural design of the power transmission line and a tower, and obtaining the lightning frequency along a power transmission line corridor; and calculating to obtain lightning current and overvoltage distribution when the power transmission line is struck by lightning through an electromagnetic transient calculation method. Natural multiple lightning strokes are equivalent to single pulse lightning strokes through an amplitude and waveform adjusting method. Then overvoltage and lightning arrester energy absorption values at the two ends of the lightning arrester body are calculated to judge whether flashover tripping happens to the base tower or not, and in this way, the lightning stroke tripping rate of the whole line is obtained through calculation. Guidance and suggestions can be provided for research and development and configuration of a lightning protection device of a lightning conductor line, sothat lightning trip-out rate parameters in the lightning protection performance can be obtained more accurately and reliably.