Abstract: In this work,it is demonstrated that since 1956 all the El Nino events occurred during the large-amplitude and continuous decrease of the earth rotation speed with an average lagging of 14 days.It may suggest that decrease of earth rotation speed probably has an important effect on the occurrence of an El Nino event.Its influence on the order of magnitudes of the weakening of equatorial ocean current velocity and the physical process of the formation of an El Nino event are also discussed.