A dynamical surface emission center model which can explain all observed phenomena in thermionic cathodes is proposed. The emission comes from such an aggregate of atoms that the moving electrons may have momentarily highest energy and/or the attraction from the nuclei to these electrons is momentarily least. Excess barium is the source for supplying high energy electrons, while the oxygen may play a favorable or unfavorable role to emission depending upon its relative quantity and relative position in the aggregate. Although strontium, calcium, aluminate, tungstate, etc. are minor factors to emission, yet they may have some effect on the evaporation, migration, and transition of electrons etc.. The non-uniformity of emission in practical cathodes proves the existence of such centers, while their dynamical character exhibits instability of emission e.g. flicker noise.