DOAS (differential optical absorption spectroscopy) is an important gas measurement system based on the Lambert-Beer law especially for the trace gas measurement. The measurement key of DOAS is the calibration of the measurement limit. The traditional calibration method is using standard gas. But the concentration value of standard gas has a large measurement uncertainty which can be large than 10% at ppt. This uncertainty is large than the corresponding specification of instruments which are calibrated. So this traditional method is invalid. A new calibration method is given which is based on spectral optical density. The relationship between measurement limit and optical density can be established using Lambert-Beer law. The optical density as a physical quantity can be measured in 10-6 or more precisely, so the corresponding measurement limit can also be calibrated in ppb or ppt. Firstly, the kinds of optical densities and the total optical density of the measurement system which be calibrated should be analyzed and calculated according to the optical structure (or optical design diagram) of the system. This step is the key of this method because the types of the standard optical densities which are necessary for the measurement of the standard optical density film on the optical density standard equipment. Meanwhile, the optical structure should be understood in order to achieve this calculation. The optical structure include the optical path, every optic cell specification, sphere aberration, astimigation distribution, etc. Secondly, the standard optical density film should be filled in the optical path of the measurement system and the corresponding optical density value is measured. Especially, the optical density film must be filled in the proper station which is confirmed at the step 1 when calculate the different type of the optical density. The error station would make large measurement error which would make large effect into the end data. Then the two above optical density can be compared and the difference of the two value can be considered as the measurement limit. This method is based on the optical measurement without the standard gas. The evaluation error sources only are the measurement error of optical density and the alignment error of the optical structure of the calibrated system. So the calibration of the measurement system can be achieved on small uncertainty, and the measurement limit can be more precise. This method is applied on an open path DOAS and the corresponding experiment data are given.