Abstract: Grids are the important parts which have important influence on the thermal-hydraulic performance in fuel assembly and the mixing vanes of grids mix the fluid. Focused on the mixing effect of the mixing vanes to fluid, Computational Fluid Dynamic（CFD） method is used to research wherever the mixing is weak inside or outside the subchannels in this paper. The results show that larger size of the mixing vanes can enhance the mixing capability of grids, and the new type design of mixing vanes with assistant vanes can reduce the asymmetry of mixing caused by main vanes only, so as to tone up the vortex mixing strength inside the subchannels. The optimized distribution of assistant vanes will not weaken the cross mixing strength between two adjacent subchannels so much, and is helpful to the development of mixing effect axially. Meanwhile, the additional pressure drop of grids has also been considered.