Abstract: In this paper, using the CDSN broad-band digital recordings, we calculated the spectrum of thedisplacements of P waves in the far-field radiated from a fault whose mode of rupture is bilateral andasymmetrical. The source parameters of moderately strong eaarthquakes are studied by the spectralmethod, and are applied to the Xiaojin earthquake with M=6.6 in Sichuan Province which occurred onSeptember 22, 1989. The results show that the fault is 8.4km long; 2.9km wide. The dynamic parameters are estemated as fallows:the seismic moment, 4.7×10¹⁷N. m F the average dislocation, 64.6cm, stress drop, 18.9×10¹⁵Pa. We also used the three-dimensional image of the focal depth distribution ofthe after shocks to prove that the earthquake malt is a strike-slip one, striking NW33°, dipping NEwith dip angle being 20°.