Abstract: The absorbed-dose to water of heavy ion beam is a fundamental quantity for heavy ion therapy. It is necessary to perform the relevant work with in-direct measurement prior to the study of the reproduction for the absorbed-dose to water of heavy ion beam. The absorbed-dose to water of a carbon ion beam, whose incident energy was 400 MeV/u and spread-out Bragg peak was 6 cm, was studied with conventional ionization method. The correction factors of polarity and ion recombination for the ionization chambers were evaluated with the incident heavy ion beam. The uncertainty components for the measurement of the absorbed-dose to water of heavy ion beam are significantly larger than that of the 60Co γ radiation, in terms of the corrections for the polarity and ion recombination of the ionization chambers. The absorbed-dose to water of heavy ion beam deduced from different ionization chambers is consistent within the acceptance of uncertainty. Based on the measurement with ionization chambers, it is crucial to conduct more intensive research activities of radiation dosimetry including the absolute measurement with calorimetric facility, with the purpose of further optimizing the uncertainty in the measurement for the absorbed-dose to water of heavy ion beam.