Abstract: A grating spectrum detecting system was designed to detect the formaldehyde concentration effectively and quickly. The spectrum detecting system mainly consists of optical system, signal collection system and computer processing system. The hardware circuits of the whole system include the driving circuit of the photoelectric magnification tube, preamplified filtering circuit, analogy and digital conversion circuit, and data collection and transmission circuit. The experiment conditions for determination of formaldehyde concentration in the air detected by MBTH spectrophotometer are investigated. The factors affecting experiment results are analyzed,experimental results for spectrum detection are obtained, the relevant conclusions are analyzed and reviewed. The transmission spectrum of the formaldehyde analytical solution，in which the formaldehyde concentration is fixed, can be correctly probed by the designed weak-light detection system.