Abstract: Stability analysis of fluid catalytic cracking units(FCCU)is an important part of operation analysis,process design and process control.For the FCCU with a highly efficient regenerator,the regenerator is composed of two stages that are characteristic of a fast fluidized bed and a bubble fluidized bed.Regenerated catalyst from the second stage of the regenerator circulates into the bottom of the first stage as well as the bottom of the riser reactor.The influence of the reactor and the regenerator on each other should be considened together in the stability analysis.Because of the complexity of the unit,the conventional technique of stability analysis such as heat generation and removal rates analysis for the CSTR with exothermic reaction is not suitable．After linearization of the dynamic mechanistic model of the FCCU with a highly efficient regenerator,stability theory is applied to the linear state space models obtained．Under the control of the inventories of the stripper and the second stage of the regenerator,the FCCU’s reactor/regenerator section is stable with in a wide range of operation conditions,because all the real component of eigenvalues of linear state space models are negative.