An experiment was designed to study 532 nm CW laser radiation on Si-doped n-type GaAs materials. The crystal axis of GaAs is 〈100〉 leaning to 〈111A〉 15°. As the laser irradiated on the materials,a diffraction-like phenomenon was observed. It was considered as the Fraunhofer diffraction formed by the reflected light from the material surface,and diffraction was developed as a new detecting method of the damage threshold for the first time. A damage threshold power density of GaAs materials,2.56 ×10⁵ W/cm²,was tested experimentally. The heat conduction equation was used to describe the interaction process,and the experimental and theoretical curves of the incident laser power density versus damage threshold time of GaAs were obtained.