Quasi geostrophic theory is the core of atmospheric dynamics, providing a method of analyzing the evolution of large scale weather systems according to the basic principles of atmospheric dynamics. This theory can help forecasters understand the 3D structures of large scale weather systems in the middle and hight latitudes, analyze and predict the development and movement of weather systems, so it is taken as the theoretic foundation of short term weather forecasts. The theory has complete theoretic system but involves some approximations that are prone to misunderstanding some of its basic problems. Proceeding from basic dynamic knowledge this paper reviews and discusses the basic concepts, basic approximations, the kernel theory of the quasi geostrophic theory and its application to the forecasting processes. The contents include definitions of the quasi geostrophic motion and quasi geostrophic approximations, the scope of the quasi geostrophic motion theory, the large scale motion in synoptic charts, correlations between the quasi geostrophic motion theory and short term weather forecasting, judgment of the development and movement of upper troughs and ridges as well as surface cyclones by the quasi geostrophic theory in baroclinic two layer model, etc.