Abstract: By using an equatorial balanced model and low order spectral method and considering the effect of the basic flow, the first-order and second-order shear, the nonlinear equation describing the winter and summer monsoon in South Asia is derived. We mainly discuss the influence of the basic flow, first-and second-order shear on the formation, transformation and intensity of monsoon in South Asia. The results show that there is an evident difference between the influence of first-order shear on monsoon in South Asia and that of the basic flow and second-order shear on monsoon in South Asia.