The instrument adopts chemiluminescence method. The basic working principle is that no reacts with O 3 to form energetic No 2. When no 2 with energy releases energy, it produces fluorescence of specific wavelength. The intensity of this fluorescence is linearly related to the concentration of No. The sample gas detection method is as follows: NO x in the sample gas enters into the instrument and is divided into two channels. One way of sample gas enters into the reaction chamber, and no in the sample gas reacts with O3 to measure the concentration of No. the other sample gas enters the conversion furnace to convert no in the sample gas, and then enters the reaction chamber to react with O3. The measured value is no 2 + no = NO x. the concentration of no 2 is obtained by subtracting the concentration of NO x from the concentration of NO x. The two-way sample gas is switched through the solenoid valve, sharing a set of detection room, and there is no system error. The photomultiplier tube transforms the light signal into electrical signal, and the concentration of no, NO x, no 2 is measured by calculation.