Abstract: Subcooled boiling heat transfer simulation of PSBT full scale 5×5 rod bundle with uniform-axial power distribution (APD) was carried out with wall heat flux partition method （RPI model）. In this study, the influence of simple support grid (SSG) on hydraulic and heat transfer characteristics under subcooled flow boiling within the last span of the bundle was concerned. It is found that the pressure drop of SSG form loss can be up to 53% of mixing vane grid (MVG). The transverse flow induced by the MVG is suppressed by the SSG, which leads a reduction of transverse velocity ratio to that of the bare bundle. Attributed to the rapid declining of transverse flow, the heat transfer decreases. At the downstream of SSG, compared with the bundle excluding a SSG at the last span, the evaporation heat flux increases by 5%, the bundle average void fraction increases by 0.006 with a rising of wall superheating by 0.3 ℃.