Abstract: This paper discussed the effect of GaAs semiconductor material thickness on the device sensitivity, and a new method for measuring the thickness of multilayer semiconductor material such as GaAs by spectrophotometer is put forward. According to the theory of wave interference, the value of wave hollow reflectivity was put into JAVA program and the thickness of multilayer semiconductor material was obtained directly. This method has many merits such as high efficiency, no damage and good repeatability. This method provides the sample semiconductor material thickness error of less than 9%, which meets the test requirement. It is an effective method for analyzing epitaxial material and improving the process of photocathode.