Abstract: In order to illuminate the characteristic of saturation of current for thermionic energy converter (TEC) operating on diffusion mode, transport equations in the interval of the electrodes applied to diffusion mode were built with amending the ionization items of the low-temperature plasma for ignited model. And the boundary conditions were selected according to the ion-richness ratio in the emitter plasma sheath. Then, the model was solved numerically based on Newton iteration method. It is found that the calculated U-I characteristic accords with lecture results. The existence of saturation current can be decided by the neutralization condition of emission and the distribution of electron potential energy can account for its principle. The orientation of potential jump in the collector sheath changes with the increase of current, which doesn’t influence the existence of saturation current.