中文摘要：

统计分析了21～23周x1和X2级以上的软x射线耀斑．统计分析结果表明，X1级耀斑占x级以上耀斑总数的52．71％，而X2级以上耀斑占X级以上耀斑总数的47．29％．无论对于X1级耀斑还是X2级以上耀斑，都是绝大多数耀斑出现在太阳活动周的下降段，而且太阳活动周强度越弱，出现在太阳活动周下降段的耀斑比例越高；太阳耀斑的强度越强，出现在太阳活动周下降段的比例越高．通过计算，X1级耀斑数平滑月均值比与之对应的黑子数平滑月均值在第21和22周均提前了1个月，而在第23周延迟了13个月．X2级以上耀斑平滑月均值相对于黑子数平滑月均值在第21周延迟了9个月，在第22周提前了1个月，而在第23周则延迟了32个月．

英文摘要：

This paper presents a statistical study on the large soft X-ray flares （X1-, X2-class and above flares） of solar cycles 21, 22, and 23 from 1986 to 2008. It is found that the number of the Xl-class flares accounted for 52.71% as a proportion of total X-class flares, while the number of the X2-class and above flares accounted for 47.29%. No matter for the Xl-class flares or X2-class and above flares, most of them occurred in the descending phase of the solar cycles. Meanwhile, the weaker the intensity of the solar cycles, the higher the ratio of the flares occurred during the descending phase of the solar cycles, and the stronger the intensity of the solar flares, the higher the ratio of the flares occurred in the descending phase of the solar cycles. In addition, the phase difference between the peak of the smoothed monthly mean number of sunspots and that of the X-class flares is calculated. The result shows that the smoothed monthly mean numbers of the Xl-class flares have a very noticeable time lead of 1 month in cycles 21 and 22 with respect to that of sunspots, while there is a time lag of 13 months in cycle 23. Moreover, for the X2-class and above flares, there is a time lag of 9 months in cycle 21, a one-month time lead in cycle 22, and a time lag of 32 months in cycle 23.