Moving mirror tilt, which causes misalignment of two reflectors, is a key issue in Fourier Transform Infrared
Spectrometer (FTIRS). Dynamic alignment system (DAS) is adopted to solve misalignment problem. High precision
detection of dynamic misalignment angle is one of the most important steps in DAS, mainly upon which the adjustment performances of DAS depend. A new measuring algorithm is presented which is based on amplitude detecting <sup></sup> between laser interference signals generated by reference laser source in this paper. Mathematical analysis is described and experiment results are shown. These results show that the measuring precision of 0.6 micro-radians of dynamic tilting angle is achieved with this method. This is a guarantee for a FTIRS to acquire high interference efficiency.