Abstract: By use of the fairly dense network of radio soundings and pilot balloons,the three dimensional structure and the formation process of the moist jet stream were shown by wind and humidity field in a case of heavy rain over north China in summer.The moist jet stream is a stream penetrating slantly through whole troposphere fxom Iow to high troposphere,which is formed as the air of the low level rises and accelerates in the moist unstable atmosphere.The distribution and the development process of the saturation and condensation area around the moist jet stream appears as if the water vapor plays a active role in the jet stream formation process.The fact,that the moist jet stream combines with the substropical jet stream near the tropopause and strenghtens the kenitic energy of the subtropical jet stream,shows that the moist baroclinic process is probably one of the essential processes in the atmospheric circulation.