In order to make full use of the lightning location system to assess the lightning trip- out risk of transmis-sion lines,the lightning current amplitude data of Dongguan area from 1999 to 2008 were used to perform a statisti-cal and comparative analysis,analysing that the fitting curves respectively obtained by the regular method and theIEEE recommended formula and the improved IEEE formula all deviate from the actual current amplitude probabili-ty distribution.Then,a sectioned fitting method of probability distribution function for the lightning current ampli-tude was proposed,followed with a correction of the continuity of functions.Moreover,the lightning trip- out rate of110kV,220kV and 500kV transmission lines for Dongguan area were calculated respectively by the regular meth-od,the improved IEEE formula and the proposed fitting method.The results were finally compared with the mea-sured ones.It is demonstrated that the proposed method helps to obtain a probability distribution function which isclosely correlated to the natural probability curve,only with a sacrifice of correlation near the sectional point; andthat the lightning trip- out rate obtained by the proposed method is consistent with the actual operational data of110kV and 220kV transmission lines,but deviates from that of 500kV transmission lines.