Abstract: In this paper,an improved supercritical water cooled fast reactor has been established.At first,reasonable fuel assembly design is obtained by studying the influences of seed fuel pin diameter and blanket coolant channel diameter to conversion ratio（C_R）.Then,void reactivity coefficient and C_R are calculated for six different core arrangements.Finally,the effect of²³⁵U and ²³⁹Pu ratio on C_R and void reactivity coefficient are analyzed for the sixth core arrangement design.The results show that,negative void reactivity coefficient can be satisfied and C_R can be increased by reducing Hydrogen to Heavy-metal ratio（H/HM） and increasing the blanket assembly numbers by properly distribution;C_R is substantially increased and more negative void reactivity coefficient can be met by reducing Pu isotopes in fuel;when 20.8% Pu enriched MOX fuel and 0.2% enriched depleted Uranium fuel has been adopted as seed and blanket assembly respectively,the sixth core program reaches C_R=1.03128 and gives a negative void reactivity coefficient,which meets the primary requirements for SCFR breeding.Then,the Physical properties of the core,such as βeff,energy spectrum,neutron flux density and power density,have been calculated and analyzed.