In order to accurately control the exposure dose, it is necessary to measure optical transmittance of lithography system accurately. A double-contrast method of optical transmittance measurement is introduced. It effectively eliminates the measurement error caused by an excimer laser energy fluctuation. In order to eliminate the measurement errors caused by diversification of excimer laser polarization, a polarizer is used in this equipment. The experimental setup is designed to measure the transmittance of an optical sample whose transmittance can be calculated accurately. The experimental results show that measurement results are consistent with theoretical calculations and the repeatability of measurement is less than 0.3%. The transmittance of optical sample is measured by the spectrophotometer, and the difference between the two kinds of equipment is 0.28% . Furthermore, the equipment has flexible application for transmittance measurement of optical system and advantage for not influenced by optical sample sizes.