Abstract: RMC is a self-developed Monte Carlo software for nuclear reactor analysis by Reactor Engineering Analysis Lab(REAL), Tsinghua University. On the basis of the self-developed subchannel modular(RMC-TH) and Monte Carlo CellTally the inner coupling interface is developed, which combines both input files and realizes the fast mesh correspondence process using the cell expansion technology for repeat structure with thermal hydraulics feedback, and it breaks through the threshold of geometrical variability and extensibility for coupled code. On-The-Fly Doppler broaden method is adopted considering the temperature effect on micro cross section, which only needs the 0K cross section library so that the memory cost can be apparently reduced. Steady state simulation analysis are performed on single rod and 1717 assembly model, and the results show the feasibility, accuracy and efficiency of the coupling methodology, so the technology roadmap and methodology foundation for the large scale and geometrically variable reactor steady-state as well as transient neutronics simulation with thermal hydraulic feedback and further neutronics-thermal hydraulics-depletion multi-physics simulation process are established.