Abstract: It is important in the reactor research and design to choose the typical lattice, which could represent the channel shape and thermal-hydraulic performance of the prototype assembly. In this study, the effect of bundle scale on thermal-hydraulic performance has been analyzed by numerical simulation method which applies to the tight arrangement spiral-fin fuel assembly.Simulation results show that compared with prototype assembly 217 rod bundle, cold wall effect and side wall effect of 19 rod bundle could be neglected. The deviations of equivalent diameter,pressure drop, center channel dimensionless mass flux and hot channel heat transfer coefficient are less than 13%. Therefore, 19 rod bundle are chosen to be the typical lattice for tight arrangement spiral-fin fuel assembly.