A preliminary improved design study of the cryogenic distillation hydrogen isotope separation system (ISS) for the fuel cycle of the ITER-FEAT, a fusion experimental reactor, was carried out based on the substantial reduction of hydrogen flow to the ISS resulting from the scale reduction from the former design for the FDR-ITER. In this study, a four-column cascade was proposed considering the 450 seconds burn / 1350 seconds dwell operation scenario of ITER-FEAT instead of the present five-column cascade design of the FDR-ITER. This proposed cascade is found to be effective in all operation phases. The impact of the optional 3000 seconds burn / 9000 seconds dwell operation scenario on the present design is also discussed in this paper. Tritium concentration in the released hydrogen stream into environment must always be controlled to be lower than the regulation limit for stack release, and the two-column system for treatment of this flow is found to be effective for meeting this requirement.