Abstract: Based on 1D second-development system code RELAP5_LEAD and 3D computational fluid dynamic code FLUENT, a multi-scale coupling code RELAP5/FLUENT was proposed by means of dynamic link library technique and user-defined functions of FLUENT. Under the single phase state, transient flow and heat transfer simulation in simple lead-cooled open circuit and transient flow simulation in simple lead-cooled closed circuit were selected to perform Code-to-Code verification between RELAP5/FLUENT and RELAP5_LEAD. The results show that the RELAP5/FLUENT simulation results are in good agreement with the RELAP5_LEAD calculation results and the multi-scale coupling code has achieved initial success. Through further improvement and fully validation, the multi-scale coupling code can be used to carry out complex 3D thermal-hydraulic phenomena analysis in the lead-cooled fast reactor.