Abstract: By studying the ewwect of Exit drift on the tearing mode instability,the influence of dawn-dusk electric field on the triggering of magnetospheric substorm is investigated.The tearing mode equation including the E×B drift term is derived and solved with a step-like electric current distribution.The Calculations show that the critical thickness of the current sheet allowed for the triggering of the tearing mode instability and the growth rate of long wave-length modes both increase with increasing dawn-dusk eleatric field.Therefore as dawn-dusk electric field is enhanced,the magnetospheric substorms are easily generated and the energy stored in the plasma sheet can quickly be released as well.