Abstract: The influences of the IMF sector boundaries of four types on the geomagnetic field activity have been studied statistically.It is found that when a sector boundary with magnetic field intensity increasing and magnetic field direction orienting southward sweeps the earth,the value of the D_st index begins to increase and achieve its maximum value in about 11 hours,which is the shortest time for the D_st to achieve its maximum in the four types of the sector boundaries which have been studied in this work.