Abstract: The thermionic energy converter is a static thermoelectric conversion device, which is the core component of the thermionic nuclear reactor. To improve the conversion efficiency of thermionic energy converters, it is necessary to test its basic performance and conduct research on the optimization mechanism of its thermoelectric performance. At present, a basic experimental device of thermionic energy converter with the flat electrode pair has been developed in China, which can be used to achieve isothermal equipotential of electrodes during the power generation experiments. However, the corresponding analysis method of thermoelectric characteristic of electrodes has not yet been established, so it is urgent to set up a reasonable testing technology for the analysis of the basic thermionic performance. By drawing inspiration from the theory and empirical formula of thermionic energy conversion, and combining with the test data of voltammetry characteristics obtained in experiments, a specialized testing technology for the thermoelectric conversion characteristics of flat electrode pair was established. Then, the tests of thermoelectric conversion of W-Mo electrode pair with the basic experimental device was conducted. The established testing technology was validated and evaluated by comparing the values of the effective work function of the tungsten emitter tested in the experimental device with the theoretical and experimental values in the literature. And the results show that the measured values of the effective work function of the emitter in this study are consistent with those in the literature, indicating that the established testing technology was accurate and reasonable. Therefore, it can be asserted that this testing technology can be used to analyze the output electrical characteristics of a flat electrode type thermionic energy conversion device. What’s more, the thermoelectric conversion performance of W-Mo electrode pairs was further studied using this testing technique. The influence of power generation parameters, including emitter temperature and cesium pot temperature, on the output characteristics of W-Mo electrode pair were analyzed through a series of thermoelectric conversion experiments. The results show that the emitter temperature has a more significant impact on the output power density of the thermionic energy converter. As the emitter temperature increases, the output voltage correspondingly increases; and within the range of 1 600-1 900 K for the emitter temperature, the corresponding optimal cesium pot temperature is 577 K, with cesium pressure of 267.7 Pa. Moreover, the maximum output power density of the W-Mo electrode pair measured using the thermionic basic experimental device is 5.6 W/cm², corresponding to the power generation parameters of the emitter temperature T_E=1 800 K, cesium pot temperature T_Cs=577 K, and output voltage V=0.7 V. This result can provide parameter reference for the application of the W-Mo electrode pair in thermionic energy converters.