Abstract: Three-dimensional flow and heat transfer model was established for TOPAZ-Ⅱ type of reactor by mature tools. The mesh was mainly generated with structured grid method supplemented by unstructured mesh. The real conditions were adopted as the input parameters. The thermal-hydraulic behavior simulation was performed with mature computational fluid dynamics software. Through numerical simulation, the three-dimensional flow field and temperature field were obtained, as well as key parameters, such as coolant mass flow rate distribution, pressure distribution, velocity distribution, and temperature distributions of moderator and reflector, etc. Through comparison, the results show good agreement with those of Russian design. And rich experience was accumulated through this effort for further thermal-hydraulic optimization of this type of reactor.