In the present paper, we describe a calibration system for OH radicals based on differential optical absorption spectroscopy (DOAS). In the system OH radicals can be produced by photolysis of H2O which is irradiated by the 185 nm light in a cavity. The produced OH radicals with a certain concentration can be detected exactly. The system consists of a xenon lamp as light source in which the light has been collimated, a 1.25 m multiple-reflection cell in which the light can reflect 60 times to achieve 75.0 m whole path-length, and a double pass high resolution echelle spectrometer that is suitable for the measurement of OH radicals (best resolution: 3.3 pm). Utilizing the system the measurement spectra and lamp spectra can be obtained for OH concentration retrieval. OH concentration can be calculated by DOAS retrieval and during the DOAS retrieval the reference absorption cross section is obtained by applying the Voigt broadening method to the absorption lines. By changing water vapor concentration, the system accurately detects OH concentration ranging from 5×108 molecules/cm3 to 1.8×1010 molecules/cm3. In the concentration range, OH concentration fluctuation is very small. For example, when the volume ratio between water vapor and pure N2 reaches 0.3 L:24.7 L, the fluctuation is just ± 4%. Taking into account the effects of absorption cross section, gas pressure in the cavity and other factors, the total systematic error of the instrument is less than 7.3%. According to the results in the paper, the system can be used for the fluorescence assay by gas expansion technology calibration in field experiments.