An auto measuring system for the thickness interpretation instrument transmission screen, using the linear CCD sensor as the detector instead of reading micrscope and using the 8253 CMOS chip to realize the auto measuring and real time displaying results, is presented. The principle and the realization of the hardware and software are discussed and the error of measurement of this system is analyzed. The feasibility and the advancede character of this system are proved and it has upper practical value and application prospect.