In this technical note, an innovative thorium-uranium–fueled fusion-fission hybrid reactor (FFHR) design that employs a dual-coolant system to enhance 233U breeding and is based on a three-dimensional engineering model is presented. The reactor consists of two kinds of modules: a water-cooled, thermal spectrum power generation natural uranium–fueled module and helium-cooled, fast spectrum fissile-breeding natural thorium–fueled modules, which are arranged alternately in the poloidal direction of the blanket. An interesting and important neutronic characteristic of the FFHR is found in this technical note: Energy multiplication is primarily determined by the uranium module parameters and is almost independent of the thorium module parameter. Uranium module design should first consider improving energy production. The 232Th neutron capture rate is primarily determined by the thorium module parameters. The uranium module parameter has almost no influence on the 232Th neutron capture rate in the thorium module. The uranium and thorium modules have weak coupling in neutronic behavior. However, with the fixed design parameters of the uranium and thorium modules, the most important influencing factor on energy multiplication factor M (the ratio of total blanket energy output and the fusion energy) and the 233U breeding rate is the fraction of the external fusion neutron source irradiated on the uranium or thorium module or the blanket coverage rate of the uranium or thorium modules. Based on this characteristic, an innovative hybrid reactor design that employs a dual-coolant system is proposed in this technical note. Uranium modules still use water as the coolant to maintain a high energy multiplication factor, whereas helium is used as the coolant for the thorium module to obtain a fast neutron spectrum to enhance the 233U breeding. The simulation results show that the helium-cooled thorium module is 2.5 times more efficient in 233U breeding compared to the original water-cooled thorium module design. Approximately 10 tons of 233U is produced after 20 years of operation for the helium-cooled thorium module design.