Abstract: The behavior of hydrogen isotope generated in fusion reactor materials is the key issue for safety and economic operation of fusion reactors and becomes an interesting field. In order to investigate the mechanism of hydrogen isotope such as diffusion, release and retention, a high-sensitivity thermal desorption spectroscopy (TDS) in combination with a quadruple mass spectrometer (QMS) was developed. A major technical breakthrough in ultrahigh vacuum (UHV), low hydrogen background, linear heating and sensitivity calibration of TDS system was made. UHV of 1×10^-7 Pa and low hydrogen background of 1×10^-9 Pa were obtained by combining turbo molecule pump and sputter ion pump. Specimens can be linearly heated up to 1 173 K at the rate of 1 to 50 K/min under the MCGS PID software. Sensitivity calibration of the TDS system was accomplished using a special deuterium leak in the detector mode of QMS second electron multiplie. The desorption sensitivity coefficient and the minimum detection limit of deuterium desorption rate are 6.22×10²⁴s^-1·A^-1 and 1.24×10^-10s^-1, respectively. The measurement was also routinely conducted on a specimen of standard, deuterium containing Zr-4 alloy maintained in the laboratory, so as to validate the TDS method.