Abstract: As one of the key components of nuclear fuel assembly, the mixing grid has an effect on thermal performance. In this paper, the single-phase flow field and temperature field of 5×5 full-length heated rod bundle with mixing van grid (MVG) and mid-span mixing grids (MSMG) are numerically analyzed and studied by CFD program, and the distribution characteristics of secondary flow field and temperature field at the outlet of the bundle section are obtained. The research shows that the downstream flow field of the grid is affected by the grid and distance, the upstream flow field of the grid has little influence on the downstream secondary flow, and the grid causes strong transverse secondary flow of the fluid. The heat exchange capability between the fluid and the heater rod is enhanced, so that the fluid temperature in the cross section of the rod bundle sub-channel is more uniform. By comparing and analyzing the experimental and calculated temperature of the 5×5 full-length rod bundle outlet sub-channel, the temperature field of the rod bundle assembly can be well analyzed by the obtained calculation model.