Abstract: With multichannel frequency modulated seismograms and calculation of the frequency spectrum, we have studied the spectral characteristics and focal parameters of small earthquakes occurring before a Ms=5.1 strong aftershock in 1979 in Tangshan. The results show that the wave spectra of small earthquakes occurring three months before this strong aftershock are obviously different from that of the other small earthquakes. Their spectral amplitude densities of highfrequecy and corner frequencies are higher. Therefore, it is concluded that the effective source radius are smaller and stress drop are higher.