A novel transparent plate thickness measurement method based on laser triangulation with the light compensation is put forward, and a design of transparent plate thickness measuring device is based on single laser displacement sensor, which is applied to correct the focal plane in excimer laser micromachining of transparent material based on projection lithography. Firstly, the relationship between bending light and transparent plate location based on laser triangulation method is built, and the relationship between displacement of scattering light spot and plate thickness is analyzed. The changing rules of measuring results caused by the distance between transparent plate and scattering base plate and the position change of scattering base plate are also analyzed. Then transparent plate thickness measuring device based on single laser displacement sensor is designed. The relationship between spot migration and transparent plate thickness is tested, and compensation coefficient of polymethyl methacrylate (PMMA) thickness measured by experiments is 0.441. Based on the theoretical analysis and experimental verification, the results show that laser displacement sensor readings are linear relation with the thickness of transparent plate, and the distance between transparent plate and scattering surface has no effect on it. The average absolute error of PMMA plate thickness measurement is less than 0.01mm, and the average relative error is 0.6%, which can meet the precision requirements for processing biochip of PMMA base by the excimer laser (KrF, 248 nm) micromachining system.