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The fitting formulas recommended by IEEE and the related departments in China are studied with the least square method， by means of the monitoring data of the lightning location system over Zhejiang in 2007 and 2008， as well as the curve fitting toolbox in Matlab. It is found that the former fits better than the latter. Through analyzing the relative errors of the calculated results of the IEEE formula and the observed values， it is found that the cumulative frequency of positive lightning current amplitude is smaller than 10%; but the cumulative frequency of negative lightning current amplitude is big in the range of -1 kA to -300 kA， with the maximum being about 38%. In response to these circumstances， the approximate function of the relative error curve fitted by mathematic software is presented， and the original cumulative frequency formula is corrected. The relative errors are significantly reduced. The application range of the formula can be enlarged from original （2 kA， 200 kA） to （1 kA， 270 kA） for positive lightning and （-300 kA， -1 kA） for negative lightning.