Abstract: In this paper,the core mode of improved supercritical water cooled fast reactor is 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（CR）.Then,viod reactivity coefficient and CR of six different core arrangements are calculated.Finaly,the influences of fuel components to CR and void reactivity coefficient are analysed.The results show that negative void reactivity coefficient can be satisfied and CR can be increased by reducing Hydrogen to Heavy-metal ratio（H/HM）,increasing blanket assembly numbers by proper distribution.CR is substantially increased and more negative void reactivity coefficient can be met by reducing PuO₂ mass ratio in fuel.when PuO₂ mass ratio reach 20.8% in MOX fuel and ²³⁵U enriched at 0.2% in UO₂ fuel have been adopted as seed and blanket assmbly respectively,the sixth core program reaches CR=1.04395 and give negative void reactivity coefficient,which meets the primary requirements for SCFR breeding.