Abstract: In this paper, We calculated and analysed the theoretical spectral amplitudes of the displacements of P waves in the far-field radiated from a rectangular fault whose made of rupture is bilateral and asymmetrical. A preliminary method for analyzing the rupture process is presented and applied to the study of the Haicheng earthquake which occurred on February 4, 1975. The results show that the rupture process of this shock is bilateral and asymmetrical along a NWW direction. The total length of the fault is 54 km. The principal rupture propagated 38 km in the NWW direction and 16 km in the SEE direction, both with velocity of about 1.3 km/sec. The source parameters are estimated as follows: the average dislocation of strike slip 117 cm; dip slip, 33cm; seismic moment, 5.2×10²⁶ dyne-cm; stress drop 22 bars and strain drop, 3.3×10^-5. The total energy amp;quot;released is 3.4×10²²ergs.