In order to overcome the slow speed of detecting trace formaldehyde in the sample gas, material consumption by chemical reaction, and the limitations of the sampling area in the detection of trace formaldehyde, a multi-wavelength characteristics method for getting the exact concentration of formaldehyde quickly was designed. According to the spectrum characteristics of formaldehyde and the main interfering gases the system chose multiple wavelengths with the minimum degree of coherence (the number of characteristic wavelengths were selected to be 3, 4 and 5), in conjunction with the corresponding groups of narrow-band filters. With the infrared light of the light source through the chamber windows and narrow-band filters, the infrared light was collected by the PCI-2TE-13 infrared detectors, and the concentration of formaldehyde in the sample gas was calculated by the characteristics spectrum absorption algorithm. In the experiments, the system analyzed and calculated the concentration of formaldehyde in four gas samples collected in the newly renovated house, building materials market, supermarkets and outdoor parks. Experimental results of the system and test results of ARCSpectro-AMIR infrared spectrometer were compared, the results show that test data above 10 mg·m-3 were close to true value by the multi-wavelengths characteristics method, and the average error is less than 5%. So the system meets the requirements of practical applications, and it has the advantages of real-time detection, not poisoning so on.