Abstract: In this paper, starting with the equation set of barotropic atmosphere with semi-geostrophic approximation, the KdV equation satisfied accurately by nonlinear Rossby waves is obtained. The solution are Rossby conical waves and solitary waves. The phase speed of waves and the relationships between the amplitude and the other parameters of waves are discussed. Additionally, it is pointed out that when the mathematical simplification was done for the atmospheric model, certain conservation law of the primitive model should be conformed as far as possible, otherwise, the solutions of the original problem would. be distorted.