Abstract: In this work, the activity microcalorimeter of tens of μW has been successfully developed. The semiconductor thermocouple is used as the temperature measuring element for micrometer. Meanwhile, the original single-cup measurement method is improved and the double cup compensation method is proposed to reduce the lower limit of thermal power measurement for micrometer. According to the principle of double cup isothermal measurement, a double cup microcalorimeter system is built, and the software of data acquisition and data processing is designed and developed. In this paper, the electrical performance and sensitivity coefficient of the double cup microcalorimeter system are calibrated, and the sensitivity coefficient of the double measuring cups and the signal-to-noise ratio of the measurement limit are obtained respectively.