Abstract: The optical absorption coefficient of KDP crystal is an important parameter to evaluate its quality. Especially, in the Inertial Confinement Fusion (ICF) facility, the amount of the optical absorption coefficient determines the energy and frequency conversion efficiency of output pulses directly. So it is need to measure it with extreme precision. Since the magnitude of optical absorption coefficient of the KDP crystal is generally 10-3/cm~10-5/cm, traditional detection methods cannot meet the measurement resolution requirement. In this paper, an interferometry method based on the photothermal displacement principle was proposed to solve the problem of measuring the optical absorption coefficient with extreme precision in connection with the characteristics of KDP crystal. The mathematical model within interferometry method of KDP crystal optical absorption coefficient was established by the integral transfer method firstly, then the structure parameters of the measuring system was simulated and optimized based on the above model, and the design parameters of the system were obtained.