Based on the electrolytic reaction and the balance equation of hydrogen isotope exchange in gas and liquid phases, the dynamic equation of impurity hydrogen is established during heavy water electrolysis. And combined with engineering practice, the calculation formula is drawn together with the calculation step and the calculation methods of all parameters while batch feeding heavy water electrolysis. These lay the necessary theoritical foundation for the research of heavy water electrolysis and the production of high-purity deuterium. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.