PM2.5 is the major pollutant in most cities of China, especially contributed significantly to the poor air quality in Beijing. This study aimed to investigate the long-term trend (2000–2015) of PM2.5, based on intensive observation and comprehensive literature investigation of PM2.5 and its chemical components. Results showed that the annual average concentration of PM2.5 generally decreased by 1.5 µg m–3 year–1 from 2000 to 2015 under the implementation of 16 phases’ air pollution control measures. In the most polluted season (winter), four change stages were found related with emission control effect and meteorological conditions: PM2.5 decreased in 2000–2008 and 2010–2013, increased in 2008–2010, and was at a high level in 2013–2015. As for detailed chemical components, OC, soil dust and typical elements (Si, Ca, Fe, Mn, Cu, Pb and K) presented a downward trend generally. EC had almost no change before 2003, increased from 2003 to 2007, but decreased after 2007; this may be caused by the replacement of fossil fuel and control of biomass emission. The continuous rising of OC/EC and SOC/OC (secondary organic carbon, SOC) in recent years illustrated the secondary carbonaceous species pollution is becoming serious. SO42– showed a slight increase from 2000–2013, but decreased obviously from 2013–2015. NO3– and NH4+ had an upward trend during the past decade. The proportion of secondary inorganic aerosol (SIA) increased at a rate of 0.7% yr–1, and it has become the major composition of PM2.5 in Beijing instead of carbonaceous component since 2009. In general, PM2.5 change trend indicated the emission mitigation measures implemented in Beijing have reduced the primary PM2.5 effectively. However, the control of secondary components should be paid special attention in order to further improve the air quality in Beijing effectively.