Based on the field experimental data obtained by lightning broadband interferometer system in Naqu area,Tibet in summer of 2002,the time-clustering behavior of lightning radiation event sequences is investigated with statistical approaches of Coefficient of variation(C_V),Fano Factor Analysis(FFA) and Allan Factor Analysis(AFA).The 33 records of cloud flashes are analyzed,the apparent fractal behavior with a high degree of time-clusterization was exhibited,the mean of coefficient of variation C_V was 5.2,the scaling exponent of AFA and FFA: α_AF,and α_FF,were1.42and 0.81with average,respectively.It was also found C_V increase to the value far greater than 1before occurrence of the initial large K event.