Flow distribution of reactor core directly affects the safety of SFR(sodium-cooled fast reactor). Largegrid-baffled header is a key component in sodium-cooled fast reactor to ensure the flow distribution of core, in which equipped with some complex structures. The existing largegrid-baffled header design may cause the non-uniformity of its internal flow distribution which will further influence the flow and pressure distribution of the core. In order to ensure the uniformity of internal flow distribution, it is necessary to investigate the flow distribution optimization design of largegrid-baffled header. An innovative flow distribution structure was developed with adding the diversion sleeve in largegrid-baffled header. Three-dimensional numerical simulations on initial and optimized structure of largegrid-baffled header were carried out by using CFD software. The results show that: the asymmetric distribution of diagrid inlet leads to uneven flow distribution at the outlet under the initial structure condition. The flow distribution uniformity of the high-speed sodium liquid entering the largegrid-baffled header header area through the main pipeline is significantly improved after passing through the low porosity diversion sleeve. The safety of SFR is further improved. It leads to the increase of flow resistance in the largegrid-baffled header, and the selection of the main pump needs to be considered. Research provides the important numerical reference for the design of largegrid-baffled header in SFR.