The invention belongs to the technical field of nuclear power plant tritium concentration measurement, particularly relates to an on-line tritium measurement method based on an in-order measurement dynamic error calculation method, which aims to solve the problem of inaccurate measurement caused by change of tritium concentration on site and save the total period time of in-order measurement. According to the dynamic error calculation method for sequential measurement, the problem of inaccurate measurement caused by the change of the tritium concentration on site is solved, meanwhile, the total period time of sequential measurement is scientifically saved, and the measurement accuracy in the shortest time is ensured.