Beijing　authorities　issued　three　red　alerts　for　heavy　air　pollution　at　6　pm　on　December　7　and　7　am　on　December　18,　2015,　and　at　1　pm　on　December　15,　2016,　respectively.　To　better　understand　the　different　causes　of　the　three　red　alerts　and　assess　the　effectiveness　of　emergency　measures　dealing　with　heavy　air　pollution,　the　PM2.5　concentrations,　meteorological　conditions,　weather　conditions,　and　air　mass　transmission　were　analyzed　during　the　red　alerts　using　data　from　environmental　and　meteorological　monitoring　stations　in　Beijing.　All　three　red　alerts　were　affected　by　secondary　transformation,　but　the　high　PM2.5concentrations　were　mainly　affected　by　meteorological　conditions.　During　the　first　and　second　red　alerts,　the　ground　in　Beijing　was　controlled　by　a　uniform　pressure　field　and　the　regional　transport　was　mainly　affected　by　southwestern　and　southern　air　masses.　During　the　third　red　alert,　the　ground　was　under　the　control　of　a　wide　range　of　low　pressure　and　affected　by　the　superposition　of　southwestern　and　local　air　masses.　During　the　third　red　alert,　the　PM2.5　pollution　was　the　most　serious.　Its　average　concentration　was　the　highest　(273.6　μg•m-3),　that　is,　1.2　times　and　1.3　times　higher　than　that　of　the　previous　two　alerts,　respectively.　The　WRF-CMAQ　model　was　used　in　combination　with　the　emission　reduction　of　each　source　to　evaluate　the　effects　of　the　emergency　measures　related　to　the　third　red　alert.　The　results　show　that　the　average　daily　emission　reduction　of　pollutants　was　678.4　t,　the　average　concentration　of　PM2.5　decreased　by　79.1　μg•m-3,　and　the　average　reduction　ratio　was　26.9%　after　the　emergency　measures　were　implemented.　The　increase　of　coal-fires,　traffic,　and　other　sources　control　the　emergency　measures,　early　alert　start-up　time,　and　inter-region　joint　prevention　and　effectively　mitigate　the　increasing　PM2.5　concentration.　©　2019,　Science　Press.　All　right　reserved.