The theory of slantwise vorticity development is used to diagnose a severe winter rainfall and snowfall process over Yunnan.Results show that the very stiff moist isentropes can lead to the development of moist slantwise vorticity and the severe precipitation may occur there;conditionally symmetric instability is another important factor responsible for this severe precipitation;the weak atmospheric stability and strong baroclinicity are favourable to the happening of severe winter precipitation over Yunnan.