An open path Fourier transform infrared spectroscopy system which measured the ambient concentrations of multi-gas in Yanshan area of Beijing during 2008 Beijing Olympic Games is presented. The quantitative analysis model used for the determination of the concentration of the unknown species is described. The measured results show that, most of the gases were decreased during the traffic controlling ([EQUATION]29%) and the period of Olympic Games ([EQUATION]34%). With the influence of background concentration, C[EQUATION]H[EQUATION] increased of 6.09% and 19.66% during the traffic controlling and the period of Olympic Games. However, the typical diurnal variation shows the emission of C[EQUATION]H[EQUATION] was reduced significantly.