Abstract: Using a nuclear high-temperature heat pipe as the external heat source, this study investigated the effect of introducing phase change material at the cold side of a segmented annular thermoelectric generator (SATEG) on its output performance. A three-dimensional thermoelectric coupling model of the SATEG with phase change material was established and solved in COMSOL to quantitatively evaluate the influence of passive thermal management on the transient output characteristics, and the SATEG was coupled with the high-temperature heat pipe to construct an integrated system whose steady-state operating performance was analyzed. The apparent heat capacity method was adopted to describe the heat absorption and release process of the material, and the latent heat of phase change was converted into an additional specific heat within the phase change temperature range. The results show that an appropriate material height exists within the investigated range: When the height is too small, the heat storage capacity cannot be fully utilized, whereas an excessively large height reduces material utilization and causes the performance gain to drop. A balance between output performance and material utilization is achieved when the material is just fully melted during the heating stage. Among the investigated cases, a phase change material height of 15 mm is appropriate. At this height, the average steady-stage thermoelectric conversion efficiency of the SATEG unit reaches 10.62%, 2.16% higher than that of the reference case without phase change material. The integrated system based on this configuration delivers 47.77 W under the optimal load. The introduction of phase change material does not affect the load sensitivity of the integrated system, and the thermoelectric efficiency is the same as that of the SATEG unit in the steady stage, indicating that the additional thermal and electrical losses introduced during cascading and integration are negligible. The effect of the material is closely related to the dynamic heat input process: During heating, it absorbs sensible heat and latent heat to suppress the cold-side temperature rise, enlarge the temperature difference, and improve the output performance; During cooling, it releases stored heat and delays the decrease of the cold-side temperature, which accelerates the decay of the temperature difference and leads to a more obvious decline in output performance. Under the condition of equal output power, phase change material also reduces the convective heat transfer requirement at the cold side of the SATEG unit, which is beneficial for reducing cooling energy consumption and increasing the net power generation in practical applications. This work provides guidance for the passive thermal management design of integrated systems coupling the SATEG with a high-temperature heat pipe.