The 2015/2016 El Niño, which was generated with smaller westerly wind burst in 2015 than in 1997, had a similar strength with the famous extreme 1997/1998 El Niño and aroused wide concern in the scientific community, the formation mechanisms of this special El Niño is worth to study. According to the three regions and different phases we divided, associated with the heat balance equation we used and the comparison we made with the 1997/1998 El Niño in this study, the formation of the 2015/2016 El Niño was clarified. The effect of VADV was relatively small when compared with the UADV and WADV in all the three regions, and the variation of WADV was decided by the variation of UDIV and VDIV. The heat accumulation during the preparation phase was determined by the unique zonal wind variation in 2014, which maintained relatively steady in 1996, and the positive heat content anomalies could only be kept in the middle equatorial Pacific because of the small decreasing of temperature tendency, which was decided by the small variation of UDIV and VDIV, around January 2015. This unique heat content accumulation during the preparation phase in the 2015/2016 El Niño provided closed UADV with 1997/1998 El Niño when the El Niño began to develop, especially in the eastern equatorial Pacific, which help to push the 2015/2016 El Niño to a comparable extreme El Niño with the 1997/1998 El Niño.