To study the causes of EI Ni(n)o Phenomenon the Extended Associate Patt Analysis (EAPA) is set up with monthly sea surface wind stress and precipitation besides Sea Level Pressure (SLP) and Sea Surface Temperature (SST) in or over the Pacific and related seas since 1979 in present paper. The results show that El Nio Phenomenon means a SST redistribution, covering almost the whole Pacific that needs a huge amount of energy provided by air-sea interaction in the most Pacific regions; It is the wester burst and wind convergence, coming from meddle latitudes directly, instead of Kelvin waves that produce the strong SST warm signal in Nino regions; Two negative SLP anomalies in middle of the North and South Pacific, most likely produced by the average air pressure decreasing effect of explosive cyclones through precipitation, and the positive SLP anomaly over the Asia-Australia land bridge, coming mainly from the south hemisphere with a little influence from the Asian continent, are the three major causes for EI Ni(n)o onset, they work together to produce anomalous wind convergence in Nino regions and wester burst in middle of the North and South Pacific that result in a warm tongue and cold pincers gearing together; Basically, during its early stage the ocean is forced by the atmosphere, while during its late stage the atmosphere is forced by the ocean.