There is a growing number of environmental pollution caused by excessive indoor formaldehyde, and in order to quickly and accurately quantify the concentration of formaldehyde gas in indoor air, a system for detecting the concentration of formaldehyde gas based on photo-elastic modulation was designed. It consists of the infrared light source, filters, elastic light modulator, and infrared detectors, and photo-elastic crystal refractive index of cyclical changes was controlled by elastic light modulator. Refractive index caused by the changes in the optical path provided a spectrum distribution function of the optical path difference. Optical path difference function of the system was derived through the HITRAN spectral database. Experiments were carried out using infrared light source combined with narrow band filters, and the transmittance of the center wavelength was more than 90%. Photo-elastic crystal is ZnSe crystal as photo-elastic light modulator, and the drive frequency of the system is 100 kHz. For three different environments at different locations, 10 groups of sample gas were collected for analysis, and the concentration of formaldehyde gas was detected using standard spectrometer and the system for comparing the test data. Experimental results show that when the concentration of formaldehyde gas is high, the system performance is good; When the concentration of formaldehyde gas is low, the signal-to-noise ratio of the system is decreased, and the detection accuracy is slightly reduced, but it still meets the design requirements.