Abstract: With a compact structure, static thermoelectric conversion can be achieved by the single-cell thermionic fuel element (single-cell TFE), which shows obvious technical advantages and broad application prospects. However, fuel mass transfer occurs due to extremely high operating temperature (over 2 000 K), which affects the performance of the single-cell TFE in different fields. With the consideration of the geometric characteristics of single-cell TFE, the fuel mass transfer model, thermionic conversion model and mechanical model were built according to the circuit connection of the single-cell TFE. Based on above models, the steady-state performance analysis code for the single-cell TFE was developed. A comparison between the calculation results with the in-reactor test data of China Institute of Atomic Energy was carried out to further verify the thermal calculation function of the code and the calculation result and it shows good agreement with the test result. After that, the steady-state performance analysis of the single-cell TFE was carried out. The study indicates that the main component of fuel mass transfer is uranium dioxide when sub-chemical dose of uranium dioxide is used. In the central channel, fuel evaporates in the relative high temperature area and condensates in the low temperature area. The axial mass transfer of the fuel increases the diameter of the central channel in the higher temperature region and decreases in the lower temperature region on both side. Under the combined action of temperature and migration distance, the position corresponding to the minimum diameter of the center channel is not at both ends, but there is a certain distance between the position and the two ends of the central channel. The axial fuel mass transfer causes the axial redistribution of the power of the fuel element, thereby the axial temperature distribution is smeared obviously, and the maximum axial temperature difference in the inner wall of the fuel decreases significantly. The risk of central channel blocking is higher with higher thermal power and smaller initial diameter of the central channel. Fuel pellets and the emitter contact in the early stage of operation due to radial fuel mass transfer. The emitter creeps under the contact, and the creep rate is affected by the axial fuel mass transfer since the contact is stronger in the area where the fuel condensate more. Under the influence of fuel axial mass transfer, the emitter creep at the range of fuel condensation is greater than that at the range of fuel evaporation so that a trend of high at both ends and low in the middle is formed in the emitter radius axial distribution.