Since the last Ms6 earthquake event in 1997 to 1998 Jiashi strong earthquake swarm, the seismicity in this region tended to quiet. Then occurred Jiashi MsS. 4 earthquake on Jan. 4,2003, and Bachu--Jiashi Ms6.8 earthquake about 20 km far from the MsS. 4 earthquake 50 days later. After that 7 strong aftershocks（Ms≥5.0） followed it, and the aftershocks of the sequence are richer and strong. By calculating static Coulomb rupture stress changes of the mainshock acting on the rupture surfaces of strong aftershocks, combining spatial and temporal characters of aftershock evolution, the quantitatively possible mechanism of the strong aftershock activity is discussed. The result shows that Bachu--Jiash Ms6.8 earthquake not only encouraged saftershock occurring, whose direction of epicenter distribution is consistent with main rupture surface, but also triggered seismicity in the conjugated direction of the main rupture surface.