Abstract: In this paper,the adaptation process in low latitude is discussed by means of the two-layer barolinic model on the equator B plane,showing that the adaptation process in low latitude is mainly dominated by the internal inertial gravity wave.The initial ageostrophic energy is dispersed by the internal inertial gravity wave,and as a result,the geostrophic motion is obtained in zonal direction while the ageostrophic motion maintains in meridional direction,which can be called semi-geostrophic balance in barotropic model as well as semi-thermal-wind balance in baroclinic model.The vertical motion is determined both by the distribution of the initial vertical motion and that of the initial vertical motion tendency,but it is unrelated to the initial potential vorticity.Finally,the motion tends to be horizontal.The discussion of the physical mechanism of the semi-thermal-wind balance in low latitude shows that the achievement of the semi-thermal-wind balance is due to the adjustment between the stream field and the temperature field through the horizonal convergence and divergence which is related to the vertical motion excited by the internal inertial gravity wave.The terminal adaptation state obtained shows that the adaptation direction between the mean temperature field and the shear flow field is determined by the ratio of the scale of the initial ageostrophic disturbance to the scale of one character scale related to the baroclinic Rossby radius of deformation.The shear stream field adapts to the mean temperature field when the ratio is larger than 1,and the mean temperature field adapts to the shear stream field when the ratio is smaller than 1.