Based on two datasets of the Simple Ocean Data Assimilation (SODA) analysis gene rated by Maryland University and the NCEP/NCAR atmospheric reanalysis,temporal a nd spatial evolutions of the available potential energy (APE) during the El Nino episodes in the tropical Pacific are studied,and possible interdecadal changes o f El Nino over the last 50 years are examined by making use of the APE.It shows that the characteristics of ENSO has an obvious shift around the end of 1970s.Si nce the end of 1970s,the El Nino variability in the El Nino phase also experienc es a c hange before and after the end of 1970s.Before the end of 1970s,prior to onset o f an El Nino event,the available potential energy anomalously accumulates in the western tropical Pacific.Then the accumulated APE propagates eastward and relea ses potential energy under the effect of free Kelvin waves,which makes the energ y anomalies occur in the eastern equatorial Pacific,and an El Nino event occurs. On the contrary,after the end of 1970s,there is no APE accumulation in the west prior to onset of an El Nino.An El Nino event develops with an APE anomaly first ly from the central equatorial Pacific as an unstable coupled air-sea mode,whic h migrates eastward and finally results in a well-defined El Nino event.