As is well known, gaseous chlorine dioxide holds great prospect for room disinfection, but the technology of on-line concentration monitoring is immature, which hinders its widely use to a great extent. Based on absorption photometry principle, using LED as a light source, the differential absorption type measuring unit with simplified single wavelength and double light paths was designed. This paper realized the concentration sensor of gaseous chlorine dioxide with a detection range of 0~9 mg/L. Various measures were taken to avoid the photochemical decomposition of gaseous chlorine dioxide in the detection process to improve the stability of the sensor. Additionally, the experiment platform was designed to calibrate the sensor. The result shows that the design can meet the demand of on-line concentration monitoring for room disinfection using gaseous chlorine dioxide, with the accuracy of better than 2% of full scale.