This paper studied the 3-D single-phase flow field in 5×5 rod bundles with spacer grids by numerical method. By using hybrid grids technique, SST k-ω model and SIMPLEC algorithm, the Reynolds averaged mass conservation equation and momentum conservation equation were solved, and the pressure and velocity field were obtained. The results show that the spacer grid leads to intense lateral flow in rod bundles channel, and the lateral vilocity submits parabola distributing raw; axial velocities are distributed uniformly in the channel, and drag coefficient decreases as inlet Reynolds in the universe degrade. Calculation results agreed with experiment data well.