Abstract: In order to non-destructively measure the radioactivity of β nuclide in a small volume container，a low-energy β micro-calorimeter was developed. The paper describes the working principle and structure of the apparatus. Its specifications were tested and verified. The uncertainty of measurement results for the heat power was also evaluated. When the input power is 500 μW, the uncertainty of measurement results is 0.96% (k=2), when the input power is 48 μW, the deviation between standard value of input power and measurement results is 4.2%.