Abstract: It is indicated that the semi-geostrophic adaptation motion and evolution motion are the two kinds of basic motion in tropics. They are separable in time and are the results of different physical processes in different phase of motion. After the gravity-inertia wave dispersion which is excited by initial disturbance, the semi-geostrophic quasi-balance state is established. In this stage, the motions are controlled by the Rossby wave (short or long wave length), and the Kelvin wave. The studies of zonal semi-geotrophic processes(long wave approximation) are the foundation of Gill model.Besides, in this paper the meridional semi-geostrophic processes (short wave approximation)are also investigated for the studies of some kinds of motions near the meritional boundary of ocean.