Abstract: In order to study the change characteristics of ambient air quality and influencing factors in Xi'an during autumn and winter,emission reduction effect assessment and characteristic radar map were used to analyze the effects of traffic flow and meteorological conditions on ambient air quality.The results showed the concentrations of primary pollutants (PM₁₀,SO₂,NO₂ and CO) in Xi’an decrease during stageⅡof two study periods compared with the same period last year.However,the concentration of O₃ increased compared with the same period last year and before control stage.Xi’an had 5 times PM_2.5 heavy pollution periodsas a result of the local pollutants accumulation and secondary transformation in stageⅡ of two study periods.Firstly,the boundary layer decreased and most of the wind was still,which was not conducive to pollutants diffusion.Secondly,high humidity enhanced the hygroscopic growth of PM_2.5.In addition,the increase of O₃ concentration led to enhancement of atmospheric oxidation,and the formation capacity of secondary organic aerosol （SOA） became stronger.The traffic flow in Xi'an decreased 67%and 72%respectively in stageⅡof two study periods,the concentration of NO₂ in roadside stations and atmospheric air stations decreased by 30% to 50%.The on-road vehicle emission was the main direct source of NO₂ in Xi'an.