Abstract: Multi-scale coupled simulation has become a trend in performing nuclear reactor safety analysis. The routine method is to combine different codes together by a common platform for data exchange and delivery. In this paper, a new thought was proposed that the analysis technique developed at relatively small scale could be extended to large scale and it was verified with the application of CFD method to perform the subchannel scale analysis. The result confirms its reasonability in a sense, however, it needs further improvement since it doesn’t account for the turbulent mixing between the adjacent subchannels properly.