In this paper, an Infrared Sulfur Analyzer is set to measure the desulphurization degree during thermal decomposition of phosphor-gypsum, which provides a much quick, convenient, continuous and accurate method. It might be considered as an effective way to study kinetics of phosphor-gypsum (or phosphogypsum) thermal decomposition (or pyrolysis). With the help of this method, the desulphurization degree and desulphurization rate of broadly constituted phosphor-gypsum samples in the temperature range of 1250-1400°C were studied. The experimental results show that the desulphurization degree increase with the rising of temperature in the range of 1250-1300°C quickly, however, this increase is not obvious when the temperature is above 1300°C. Besides, the corresponding regression equations are obtained. The decomposition degree of phosphor-gypsum could be judged according to the regression equations mentioned above.