The invention discloses a lightning current amplitude cumulative probability distribution curve fitting method. Based on N statistical scatter points, by using an IEEE recommended expression as a prototype function and using the least squares as a criterion, in combination with a median current method and a Levenberg-Marquardt algorithm, a lightning current amplitude cumulative probability distribution curve fitting method is obtained which enables a fitting curve and the statistical scatter points are closest to each other and have the lowest residual sum of squares. The method can fit an accurate lightning current amplitude cumulative probability distribution curve, objectively reflects the lightning law and characteristics, and is used for evaluating the lightning protection performanceof a transmission line and a tower, and guiding the lightning protection engineering design.