The propagation features, stabilities and dynamical characteristic structuresof coupled Kelvin inner modes and second order Rossby inner modes are studied using asimple tropical coupled air-sea model in this paper. It is shown that there is mechanismof selecting scale and frequency in the tropical air-sea system. The effects of air-seacoupling are mainly on the large-scale modes and nonuniform These effects make the frequency of Kelvinmodes decrease that of Rossby inner modes increase,and even excitethe eastward propagating Rossby inner modes. These effects make the unstable development of Kelvin modes and result in the decay of Rossbt modes. The effects of atmo-spheric damp are opposite to those of air-sea coupling. The oceanic damp only makesthe wave amplitudes decay Simuitaneously, this paper shows the dynamical charac teristicstructures of air-sea coupled system and the phase relationships between the atmo-spheric and oceanic wave component.