The aim of this study is to deeply search and understand the performance of the coupling system composed of an alkali metal thermoelectric converter and a thermoelectric generator. The main methods are to update the model of the coupling system, which includes the main irreversible losses existing in subsystems and coupling system, and obtain the analytical expressions of the power outputs and efficiencies of two subsystems and coupling system. The main novelties are to discuss the influences of the thickness of the beta"-alumina solid electrolyte on the whole performance of the system and solve the matching problem between two subsystems. The main contents of this study include that the influences of the thickness of the electrolyte membrane, current density of the alkali metal thermoelectric converter, and electric current of the thermoelectric generator on the power output density and efficiency are discussed and the maximum power output density and efficiency of the system are calculated and compared with those of the single alkali metal thermoelectric converter and the existing coupling models. The results obtained reveal the advantages of the present model. The whole performance of the coupling system can be optimized and the parametric selection criteria of several key parameters of the coupling system operating at optimum states can be provided, When the thickness of the electrolyte membrane is optimally selected, the maximum efficiency of the present model attain 34.6% and increase about 10.4%, compared with those of the existing best coupling system. The waste heat released by the alkali metal thermoelectric converter can be efficiently utilized by the thermoelectric generator.