Lightning current amplitude cumulative probability is a necessary parameter to characterize the frequency of thunder activity as well as calculate the lightning strike flashover.Using data from lightning location monitoring system in Shaanxi province,this paper analyses the lightning current data during 2009-2012 in Yan'an area,and compares various calculation formulas of cumulative probability for lightning current amplitude.Then the density formula is introduced,and an example is given to show the difference between the calculated and measured cumulative probability of lightning current amplitude.The least square curve fitting method of CFTOOL in MATLAB is employed to find the best fitting parameters,therefore a more precise calculation formula for lightning current amplitude cumulative probability is established.The lightning current data in 2013 is used to verify the proposed formula.Results show that the average current amplitude of positive lightnings is higher in value and less concentrated in distribution than that of negative lightnings.The distribution curve of lightning current amplitude cumulative probability is relatively flat for positive lightnings and steep for negative ones.As for the simulation results by various calculation formulas,the regular formula has the biggest error,while the formulas recommended by IEEE Std and CIGRE have similar variation trends and shapes compared with measured curves.The fitting is best with parameter α being 36.04 and β being 4.349;the fitting error is ranged in -0.025 to 0.018 when the lightning current (I_c) is from 0 to 150 kA,and decreased to close to 0 when I_c is more than 150 kA.Thus a more precise formula is established to calculate the cumulative probability of lightning current in Yan'an area,which is then verified through test.