Abstract: The method for high concentration D₂O concentration determination has advantages of rapid, convenient, using less sample and direct determination without pretreatment. The determined concentration rang of D₂O is 99.92%～99.67%. For 99.80% D₂O concentration, the RSD is better than 0.001%. The infra-red absorption spectra method for high concentration heavy water was used in one heavy water research reactor. It could quickly reflect the heavy water concentration changes and provide important parameters for reactor operation.