Abstract: To investigate the instability of flow in rotating cavities, the flow in a heated rotating cavity with cool air axial throughflow was numerically studied, based on the time-independent incompressible equations in rotating system, and low Reynolds K-ε turbulent model. The flows in the cavity, affected either by the Coriolis force, by the centrifugal-buoyancy force, or by both of these two forces were calculated. The results show that the Coriolis force acting as a resilience force does not lead to flow vibrating; the centrifugal-buoyancy force is the key factor causing flow instable, the Rayleigh_Benard convection flow induced by the centrifugal-buoyancy force is blended with the forced convection flow in the cavity, and it develops from stable to unstable as buoyancy force increases. With the coaction of these two forces the flow instability is enhanced, cyclonic and anti-cyclonic circulations can be observed circumferentially.