Abstract: In this paper, by using rigorous field analysis, a coupled-cavity（ＣＣ） chain filled with plasma has been analyzed. How the hybrid wave between the cavity mode and plasma mode is formed has been studied. The periodical CC chain filled with plasma demonstrates periodical TG modes with a cutoff frequency of zero. When the plasma density increase to a large scale, the cavity mode of the CC chain overlaps the TG mode, these two modes couple with each other and form the hybrid modes. In the case of hybrid modes, the "cold " bandwidth and the "warm" bandwidth expand, and the coupled impedance increases about 5 times larger than that of the vacuum. As a whole, the slow wave characteristics are improved substantially due to the formation of the hybrid mode.