Abstract: The widely construction and operation of commercial nuclear power plants, plays an important role in settling the problems of electricity supplying and environmental pollution. However, it hardly meets some potential applications, such as isolated islands, small bases and space propulsion. Therefore, small sized, portable nuclear reactor systems, covering a large scale of fission power, are in great need in future. Considering the merits of high-level outlet temperature and outstanding nuclear safety embedded in pebble bed reactor, a nuclear reactor with thermal power of 5 MW was designed, which was coupled with closed Brayton cycles. Meanwhile, the essential system parameters, general physical and thermohydraulics features were provided in the paper. It is indicated that the energy conversion efficiency is nearly 35.2% with the system specific mass of 6.14 kg/kWe. The excess reactivities in the beginning of lifetime and end of lifetime are 4.88 and 2.28 , respectively, which are able to achieve the duration of 10 years. Under the normal operation, the helium temperatures in the inlet and outlet of reactor are 868.7 K and 1 295.8 K, respectively. At the same time, the maximum temperature in fuel pellet comes to 1576 K, which is below the limiting value of 1600 K.