Abstract: This paper proposes a multi-layer design of fuel assemblies for the fast zone of the SCWR (supercritical water reactor) mixed core. Both neutron-physical and thermal-hydraulic behavior of the multi-layer fuel assembly were analyzed using the MCNP code and the sub-channel analysis code STAFAS, respectively. Besides, the sensitivity analyses of some parameters such as pitch-to-diameter were investigated. The results show that with the proposed multi-layer structure, the fast zone of the mixed core has breeding capability and gets a negative void reactivity coefficient of coolant; a smaller pitch-to-diameter leads to a higher conversion ratio, however, it also results in a higher nuclear spots factor. It is possible to adjust Pu enrichment for improving axial power uniformity effectively and decreasing nuclear spot factor in seed fuel assembly.