Abstract: In the tropical air-sea coupled system,the high frequency of fast coupled waves is meaningless for the studying of the ENSO events.It can be inferred that this kind of fast waves is caused by the high frequency equatorial Rossby waves in the atmosphere.In order to prove this,the high frequency disturbances in the atmosphere in the tropical air-sea coupled system are filtered out,and only the low frequency disturbances which are influential in the long period process are reserved.The results further show that the equatorial Rossby waves in the ocean play a decisive part in the long-term evolution of the tropical air-sea coupled system,which provides a physical foundation for the designing of a model to study and forcast the ENSO events with much less calculating amounts.