A Chernin multipass absorption spectroscopy system is combined with the photochemical simulation smog chamber and applied to the detection of trace gases in the smog chamber. The system accuracy and reliability are proved with NO2 detection. The NO2 measurements of the system and the results of Thermo 42i NO-NO2-NOx analyzer have a good linear relationship. The system is applied to online detection of SO2 during β-pinene/O3/SO2 system chemical reaction in the smog chamber and with 36 m absorption path length, the SO2 detection limit of the system is 22.2×10-9. The result demonstrates that the Chernin multipass absorption spectroscopy system has the capability of quantitative measurement and can be used in online detection of trace gases during photochemical reactions in the smog chamber.