Abstract: Based on the analysis of seismic data from 1970-1975 of Hongshan Seismological St-ation, it is found that most of microearthguakes occured before large aftershocks show anomolous wave forms.We have analysed their characteristics and statistically discussed its possibility of being as precursory information before large aftershock.The results have shown that epicenter of such small aftershocks is near that of coming large aftersh-ock, that the time interval between anomalous shock and strong aftershock ranges from several minutes to days, and that the relation between the magnitudes of small aftersho-cks and coming large aftershock is not obvious.Finally, the possible mechanism produc-ing anomalous wave form has been discussed in stress field and medium of focal region.