Abstract: In order to achieve accurate measurement of nuclear purity and radioactive activity of medical isotopes, a set of microthermal apparatus has been designed according to the principle of calorimetry. This device consists of calorimetry system, temperature control system and measurement and control software. The performance of the device has been tested and verified by experiments. The results has shown that the baseline potential of isotope calorimeter is stable and reproducible. The minimum detection limit can reach 3.1 μW, which can realize the full coverage of medical isotope radioactivity measurement. In the measurement range of 10 μW~1 mW, when the input thermal power is less than 100 μW, the maximum deviation between a single measurement result and the standard value of the input thermal power is less than 1.5%. When the input thermal power is greater than 100 μW, the deviation between the measurement result and the standard value of the input thermal power is less than 0.5%. Using this device, the activity of medical isotope ¹⁴C samples has been successfully measured. The measurement results are consistent with the standard values within the range of uncertainty. This method does not produce radioactive waste and chemical waste and does not destroy the sample with simple and convenient operation. At present, the thermal equilibrium time of the device varies greatly with the different packaging materials of the sample, which limits the application of short half-life isotope activity measurement. Subsequently, the adaptability and detection limit of the device can be further improved by studying and measuring the preheating and insulation device.