Abstract: Pellet bed reactor (PeBR) using TRISO coated fuel particles, which is based on the technology of high temperature gas-cooled reactor, is an ideal space heat source. Without the limitations of normal fuel element cladding temperature and phase change, the outlet coolant can reach very high temperature. According to the specific space power requirements, gas coolant can choose hydrogen or inert gases such as helium and xenon. Considering its small molar mass and high specific impulse, hydrogen is usually used in nuclear thermal propulsion. Based on investigating the development of current space nuclear power systems, the applications in space nuclear power and nuclear electric propulsion are focus points in the paper. The aspects of conceptual designs, technical characteristics, and structure arrangement of the core and energy conversion system for pellet bed reactor were introduced briefly. Results indicate that pellet bed reactor coupled with Brayton cycle is suitable for megawatt power demands and specific power of the system can surpass 100 We/kg with obvious economic advantage, which has broad development prospects in the application of space technology.