Dynamic Alignment System (DAS) is one of the key technologies in space-borne Fourier transform spectrometer. The misalignment angle detecting is a key step of DAS. The working environment of space-borne Fourier transform spectrometer requires high precision and large scale angle detection. An algorithm is presented based on instantaneous laser power detecting, which meets the requirements of the high precision and the large scale well. Furthermore, the key steps and experiment results are introduced. Experimental results show that this method is able to achieve the precision of 0.03 rad and angle scale exceeding ±π,which offers a guarantee to space-borne Fourier transform spectrometer to receive high-quality interferograms of the infrared radiation from the earth and the atmosphere.