The instrument adopts ultraviolet fluorescence method. The basic working principle is as follows: the zinc lamp emits ultraviolet light, which is filtered into monochromatic light (wavelength is 214nm) and focuses on SO 2 reaction chamber. SO 2 in sample gas absorbs ultraviolet light and produces energy level transition. SO 2 changes from ground state to excited state. Excited state so 2 is unstable. Fluorescence with center wavelength of 330 nm is emitted instantly. The intensity of fluorescence and so 2 are increased The concentration of SO 2 is directly proportional to the concentration of SO2, and the photomultiplier tube transforms the optical signal into electrical signal. In order to eliminate the measurement error caused by the fluctuation of the light path, two monitoring channels are used in the optical path, one is used as the sample gas light path to measure the excited fluorescence intensity, and the other is used as the reference light path to monitor the change of the light source of the zinc lamp, so as to automatically compensate for the change of the light source.