Abstract: The development process of nonlinear baroclinic waves and frontogenesis on the mean flows of the four seasons are studied by using a three dimensional semigeostrophic model.The calculating results show that the development of baroclinic waves on the mean tonal flows which are functions of height and latitude can cause cold and warm fronts.The disturbance development is the most intense in winter.and is the weakest in summer.The fronts caused by baroclinic waves is the strongest in winter and the weakest in summer correspondently.