Highresolution data from numerical model simulation are used to study the ageostrophic characteristics of shortterm process of East Asian subtropical westerly jet in summer.The results show that jet represents positive ageostrophic area,the maximum lies to the north of jet center region and negative areas mostly lie close to the entrance of jet stream.Ageostrophic wind shows anticyclonic shear to the south of jet and in the entrance area while cyclonic shear appears in the north of entrance and exit areas.Ageostrophic wind indicates the change of jet intensity and geostrophic deviation is closely related with shortterm change of jet strength.The change of maximum deviation wind occurs 6 h earlier than the change of horizontal wind.Distribution of ageostrophic vorticity takes on mesoscale characteristics while positive and negative values appear alternately.Mesoscale vorticity extremums mainly locates in 3 regions:Taihang Mountains,Changbai Mountains and Honshu Island(Japan),all of which correspond to mesoscale terrain over 1 000 m.Maximum areas of ageostrophic vorticity of upper air jet correspond to mesoscale terrain.Mesoscale rainfall areas correspond to dense extremum areas of ageostrophic vorticity.