Abstract: Radioactive isotope decay gives off heat, which is an important basis for calculating the activity of radioactive isotope. In order to measure the heat emitted by tritium decay, the detection limit test, working curve calibration and tritium sample measurement verification of a low detection limit microcalorimeter of μW grade were carried out. The measurement results show that the detection limit of microcalorimeter is 1.28 μW, i.e. 38 mCi tritium. In this paper, the output thermoelectric potential U-input thermal power P of microcalorimeter was calibrated in the range of 165-5063 μW (5-50 Ci tritium), and the calibration relationship obtained is U（μV）=0.141P（μW）, corresponding to the relation between tritium activity and output thermoelectric potential A（Ci）=0.213U（μV）, and the liner correlation coefficient is greater than 0.999. The standard deviation of repeated measurement of tritium samples was 1.14%, and the deviation from PVT method was 2.45%. In order to ensure the consistency of measurement, the calorimeter used in this paper reduces the fluctuation of measurement environment through water bath and vacuum environment. The calibrated calorimeter can measure the activity of tritium samples without damaging their integrity. Compared with other tritium measurement methods, calorimetry is more suitable for the measurement of solid tritium samples and has great engineering application value.