As one of the important components in PWR cores, spacer grids can maintain the position of the rods during operation as well as promote fluid motion. The overall physical phenomena and local effects introduced by spacer grids can be accounted for the crossflow mixing model. Based on the in-house thermal-hydraulics (T/H) subchannel code SAPRE, the four improvements are developed:1) The distributed resistance method (DRM) is applied to develop the crossflow mixing model in order to reflect mixing vanes grid’s contribution to crossflow mixing in rod bundles. 2) By taking more detailed grid parameters into account, the loss coefficient of grid is calculated based upon form and friction losses. 3) The Gunter-Shaw frictional relationship is established to simulate the transverse resistance caused by the spacer mixing vanes. 4)Improvement of lateral momentum equation. Due to the presence of the mixing vanes, the crossflow is increased, and the domination of axial flow in the lateral momentum equation is not appropriate. The momentum changes caused by the crossflow on the flow term should be considered. All these four models are implemented in the subchannel code SAPRE. The improved SAPRE code is validated through Hassan 5 × 5 Particle Image Velocimetry experiment and PSBT benchmark. Numerical results demonstrate that SAPRE has large advantages in accurate prediction of local T/H parameters in rod bundles, which is of significance to safety and economy of PWR.