Abstract: The interaction between tropical intraseasonal oscillation(ISO) and El Nino has been paid more attentions.But El Nino events just occur after an interval of several years,which is different from that tropical intraseasonal oscillations always exist in the atmosphere.Therefore,it is important to know how tropical intraseasonal oscillation excite the El Nino event.The data analyses show that not only the kinetic energy of tropical ISO decreases abruptly in association with the occurrence of El Nino,but also that the interannual anomalous variation of tropical ISO intensity is closely related to El Nino event.These mean that the tropical intraseasonal oscillation plays an important role in exciting El Nino through its interannual anomalous variation.A simple air-sea coupled model study shows that the periodical self-excited oscillation can be produced in air sea coupled system,but the pattern is different from ENSO mode.When there is external (atmospheric)forcing with interannual time scale,a coupled mode which likes ENSO mode will be excited in the air-sea coupled system.Therefore,the dynamical analysis has consistent result with the data nanlyses:The interannual anomalies of intraseasonal oscillation in the tropical atmosphere play an important role in exciting El Nino event.