Abstract: The theoretic model of MCORGS code,which is used in this paper to evaluate the effect of the volume ratio between uranium and water（VR） on a fusion-fission hybrid for energy production,is introduced.It is showed that at natural uranium case,the VR of 2:1 is preferred,since it can reach a comprise between energy multiplication and tritium breeding ratio,and the neutronics performance of the system can maintain more than 100 years.At the case of spent fuel from PWR,there are more room for selection of VR.Both energy multiplication and tritium breeding ratio will improve,however the fissile material breeding capacity will degrade.