Abstract: Based on point source dislocation model in layer medium, the focal mechanisms were determined by fitting the observed data with the biggest amplitude ratio of P and S waves calculated from synthetic seismogram.In inversion the digital data in Beijing network, Hebei network, and some other data observed by Xinghe, Jining and Baochang Seismostation in Inner Mongolia were used, and 145 focal mechanisms of small earthquake are inversed with the vertical maximum amplitude of P and S wave.The dominant orientation of the results is consistent with the direction of tectonic stress field in North China.It is significant that there was some abnormal change before the strong aftershock of 1999 in the consistent parameter, which is the sum of the angles between P, N, T axis in focal mechanisms and the corresponding axis in tectonic stress field.