Abstract: In order to meet the needs of on-site calibration of tritium monitor, the on-site calibration technology of tritium monitor is studied. According to the calibration method recommended by international standards, a portable calibration device was established, and the reference tritium gas for calibration was diluted and prepared by mass flow method. In order to solve the influence of on-site environment and other factors on the calibration process, a reference ionization chamber is introduced into the calibration circuit to calibrate and test the on-site tritium monitor. On the portable calibration device, reference tritium gas with different activity concentrations can be prepared for the calibration of different ranges of tritium monitors. After calibration, the calibration factor of M347 tritium monitor is 1.09 and the relative expanded uncertainty is 6% (k=2). The portable calibration device for on-site calibration can prepare tritium gas with three activity concentrations of different orders by using a tritium standard source with known concentration. The calibration of tritium monitor has good operability, easy popularization, and high accuracy.