Abstract: As a fusion driven subcritical system (FDS), a fusion-fission hybrid reactor (FFHR) is of good characteristics in neutronic physics to fulfill the functional needs for energy generation, tritium breeding and nuclear waste transmutation. The COUPLE code was used to study the effects of the volumetric ratio of uranium to water and the neutron multiplying material in the fission blanket on the efficiency of the fusion neutron source. The results show that the harder the neutron energy spectrum in the fission blanket is, the higher the efficiency of the fusion neutron source is. The results also indicate that adding neutron multiplying material beryllium properly in the blanket is suitable for enhancing the efficiency of external neutron source. The progress achieved in this study has gained some insights of further improving the FFHR conceptual design.