In this study, a thermoelectric energy harvesting device applied with extremely large temperature variation (from + 100 degrees C to -50 degrees C) for space application is presented using phase change material (PCM) for thermal control and storage. Aiming at developing a high-performance PCM to fill into the heat storage unit (HSU) of the device, the thermal conductivity, leakage and thermal reliability of paraffin-based composites with different loadings of expanded graphite (EG) were investigated. Results showed the form-stable paraffin/EG composite has enhanced thermal conductivity and maintainable latent heat storage capacity after repeated thermal cycling test. Furthermore, the prototype device of thermoelectric harvester was developed, where the proportion of EG in the PCM is key to balance the heat storage capacity and the heat transfer rate. Both simulation and experiment are used to evaluate the performance of the harvester. The experimental evidence verifies that the thermoelectric harvester with paraffin/5 wt% EG composite owns the largest total energy output and the most portion of high-grade electrical energy.