Abstract: The purpose of this paper is to investigate the relations between the formation of typhoon and the jet at the middle-lower level. The results of the analysis of 11 cases show that there is a stronger jet on synoptic scale at the middle-lower level on the southern side or northern side or both side of ITCZ, before the formation of most of typhoon(8/11) in ITCZ. After the jet at the middle-lower level become Stronger and the cyclonic shear of the basic current of ITCZ increases, the disturbances in ITCZ will develope into typhoons. The kinetic energy required for the development of these disturbances is probably transfered mainly from the kinetic energy of the basic, current with stronger cyclonic shear. The strengthened jet at the middle-lower level on both side of ITCZ may be served as an index to forecast the formation of typhoon in ITCZ.