Beijing,　the　capital　city　of　China,　has　achieved　remarkable　progress　in　terms　of　an　improvement　in　air　quality　under　strict　control　policies　in　the　past　10　years　from　various　sources.　In　this　paper,　the　characteristics　of　fine　particulate　matter　(PM2.5)　and　O-3　in　January　2013　and　2018　in　Beijing　are　discussed　on　the　basis　of　daily　sample　analysis　and　hourly　monitoring　data.　It　was　found　that　the　PM2.5　pollution　for　the　month　of　January　in　Beijing　has　been　greatly　curbed.　The　SO42-　concentration　and　proportion　of　PM2.5　decreased,　while　the　proportions　of　NO3-　and　NH4+　increased.　Organic　matter　represented　the　major　component　during　the　two　periods　with　the　proportions　of　31.7%　+/-　8.2%　and　31.4%　+/-　9.8%.　The　results　of　the　Hybrid　Single　Particle　Lagrangian　Integrated　Trajectory　(Hysplit)　model　and　Potential　Source　Contribution　Function　(PSCF)　method　showed　that　air　mass　from　southern　nearby　regions　accounted　for　34%　and　10%　in　2013　and　2018,　respectively,　which　was　closely　related　to　the　pollution　period.　Thus,　the　input　direction　of　air　mass　in　January　2018　was　more　conducive　to　the　diffusion　of　pollutants.　Modeling　results　of　the　Weather　Research　and　Forecasting　model　(WRF)　coupled　with　Comprehensive　Air　Quality　Model　Extensions　(CAMx)　indicated　that　the　contribution　of　industry　sources　to　PM2.5　and　O-3　decreased　from　2013　to　2018,　while　mobile　sources　increased.　This　was　mainly　due　to　the　different　control　policies　on　various　emission　sources.　In　terms　of　O-3　sources,　more　control　measurements　should　be　taken　on　volatile　organic　compounds　(VOCs)　due　to　its　prominent　effect　on　O-3　concentration　in　both　periods.　The　reduction　in　emissions　and　the　meteorological　conditions　both　contributed　effectively　to　the　sharp　decrease　in　PM2.5　concentration.　However,　the　change　in　weather　conditions　had　the　greater　impact　on　the　decrease　in　PM2.5　concentration,　while　the　reduction　in　emissions　was　weakened　as　a　function　of　this　change.