Abstract: Under the barotropic and non-divergent conditions, the barotropic instability at the discontinuous surface between the easterly and the westerly current in the tropical latitude is investigated. Conclusively, under these conditions, the perturbation at the interface is always unstable. By the method of small parameter expansion, the non-linear problem of the development of the disturbance around the interface is calculated to the second order approximation and the result shows that non-linear process induces harmonic waves of shorter wave length. The deformation and the growth of the disturbance around the common surface both proceed fast along with time. At the end of half a day, there is some slight deformation of the common surface, and in the neighbourhood of it only very weak closed circulation develops. But, at the end of one and half days, the interface deforms remarkably, the domain of the circulation expands to anextent of 300 kms, and the shorter wave dominates. At the same time, the cyclonic circulation in the easterly current on the north side of the common surface achieves sufficient development, and the wind velocity also intensifies notably.