The paper applies the method of correlation coefficient of spectral amplitude to two mid-strong earthquakes in northeast China． The results show the mean correlation coefficient of spectral amplitude of the Xiuyan MS5． 4 foreshock sequence in 1999 is 0． 90，and the Qianguo MS5. 8 earthquake swarm is 0． 88． It suggests that both earthquake sequences have the feature in focal mechanism similarity，and have the background condition of strong earthquake． This result conforms to the former research． In addition，the correlation coefficient of spectral amplitude of Xiuyan MS5. 4 aftershock sequence has a trend of reduction． It is worth noting that the correlation coefficient of spectral amplitude is larger before the five strong earthquakes in the Qianguo earthquake swarm． But it becomes smaller afterward，which means focal mechanism similarity has been less． The findings give us a meaningful enlightenment in determining the trend of the medstrong earthquake swarm．