The non-dispersive infrared absorption spectrum method is one of the important sulfur dioxide concentration measurement method. If the soot particles attach on the protection windows surface, the original measurement light intensity is attenuated because of the absorption and scattering of the soot particle and the sulfur dioxide concentration measurement accuracy is decreased. The protection window pollution question caused by the monodispersion soot particles is discussed in the paper. The numerical simulation results show that the sulfur dioxide concentration measurement accuracy is related to the protection window pollution. The solution method is brought out and the measurement accuracy can be improved.