Abstract: Aiming at a safe, economic and mature nuclear heating system, a low pressurized heating reactor HAPPY200 was developed. The general scheme, key parameters, core scheme, thermal-hydraulic design, structure scheme, main process system scheme, equipment scheme and safety evaluation of the HAPPY200 were designed and analyzed. A safety system based on large volume tank was used by HAPPY200, which provided passive safety for reactor system. The technical scheme of HAPPY200 has the characteristics of high security, simplified system, mature technology, short construction period, low cost of operation and maintenance and high quality of heating. It has a broad market prospect and market competitiveness. At present, the conceptual design of HAPPY200 was completed and the site of demonstration reactor was identified. Engineering design is being carried out.