Abstract: With the development of many new types of reactors, the core design will be more compact. In order to ensure the safety of the reactor core, it is required to calculate the distribution of thermal parameters in the reactor core as accurately as possible during the design phase, which requires core multi-scale coupling research for specific transient state conditions. Based on the existing subchannel code COBRA-EN, the dynamic link library technology was used to couple it into the fluid dynamics code FLUENT, and the COBRA-EN/FLUENT coupling code suitable for core multi-scale calculation was developed in this paper. Further, through some examples of rod bundle channel with a chamber, the calculation accuracy of the coupling code under steady state and transient state conditions was tested respectively. The results show that the coupling between COBRA-EN and FLUENT is effective and reliable. The results of this study will provide reliable tools for the design and safety analysis of new cores.