Environment materials play important role in our daily life. It includes so many types. NO2 is one of the most important gaseous pollutants and NO2 concentration measurement system is one part of the CEMS system. There have many NO2 concentration measurement methods and NDIR is used commonly. The received angle of the measurement light is gradually decreases while the distance between soot and the receiver becomes larger; so the true infrared light intensity is smaller than the theoretical infrared light intensity and NO2 concentration is less than the actual concentration. To reduce the soot position influence, soot position influence on NO2 concentration measurement using NDIR is studied. The influence of the different soot position is analyzed. The simulation results show that the different soot position can influent the NO2 concentration accuracy. The solution method on decreasing the soot poison influence is brought out and can improve the NO2 concentration accuracy.