Abstract: Numerical analysis is carried out for the effects of grid spacer on the forced convection of supercritical water in the sub-channels in supercritical reactor core with commercial CFD code using unstructured polygonal mesh. The results show that the grid spacer has great effects on the heat transfer characteristics of supercritical water within sub-channel. The grid spacer with flow-blockage disc can neutralize the aggravated flow chocking effect by blocking and guiding the flow around the sub-channel center and enhance the local heat transfer near the narrow gap region. Obvious swirling flow is created downstream of the grid spacer with split-vans and the heat and mass transfer between adjacent sub-channels is thus enhanced. The swirling flow can also induce velocity deficits and adversely affect the local heat transfer inside a sub-channel. The Re number has obvious effects on the local heat transfer downstream of the grid spacer with split-vans, and the improved heat transfer performance downstream of the grid spacer with split vanes is more pronounced for the higher Reynolds number case.