In the industrial production of metallurgy, petrochemical, chemical and other fields, a large amount of flammable, explosive, toxic and harmful multi-component hazardous gases (such as CO, CO2, SO2, NO, NH3, etc.) are generated, which can cause accidents. However, the existing online monitoring and warning systems cannot meet the requirements of high integration, low interference and high sensitivity. This paper designs corresponding circuits and algorithms for the factors that affect the detection results, such as weak signal, light source modulation, gas signal cross interference, temperature control, etc., and conducts experimental verification. The experimental results show that the weak signal extraction technology based on the lock-in amplifier circuit, the PWM wave modulation technology based on the MOS tube driving circuit, and the concentration inversion model based on the fusion algorithm proposed in this paper can realize the online monitoring of five kinds of hazardous gases (CO, CO2, SO2, NO, NH3), with a detection limit of 0.01%, avoid the cross interference between various gases, and are suitable for the online analysis and warning system of hazardous gases in industrial production, which have a broad application prospect.