In this paper, we propose a preliminary design for a fusion-fission hybrid energy reactor (FFHER), based on current fusion science and technology and well-developed fission technology. Design rules are listed and a primary concept blanket with uranium alloy as fuel and water as coolant is put forward. The uranium fuel can be natural uranium, LWR spent fuel, or depleted uranium. The FFHER design can increase the utilization rate of uranium in a comparatively simple way to sustain the development of nuclear energy. The interaction between the fusion neutron and the uranium fuel with the aim of achieving greater energy multiplication and tritium sustainability is studied. Other concept hybrid reactor designs are also reviewed. Integral neutron experiments were carried out to verify the credibility of our proposed physical design. The combination of the physical design with the related thermal hydraulic design, alloy fuel manufacture, and nuclear fuel cycle programs provides the science and technology basis for future development of the FFHER concept in China.