Abstract: Since the 12th Five-Year Plan,the government of China has promulgated a series of air pollution prevention policies and implemented corresponding air pollution control measures to improve environmental air quality.To study the variation of ambient air quality during the 12th Five-Year Plan (2011-2015) to 13th Five-Year Plan (2016-2020) in Tibet and evaluate the implementation effects of air pollution prevention and control measures,the variation of concentration of six principal air pollutants and the average percentage of days meeting ambient air quality standards in 7 cities of Tibet from 2011 to 2020 was analyzed.The results showed that with the implementation of a series of air pollution prevention and control measures,the ambient air quality in Tibet Autonomous Region improved significantly.Compared with the 12th Five-Year Plan period,the average percentage of days meeting ambient air quality standards increased from 97.5%±2.0% to 99.3%±0.4%,and the annual mean concentration of SO₂, NO₂, PM₁₀, PM_2.5 and the annual 95th percentile concentration of CO decreased.The concentration of CO, PM₁₀ and PM_2.5 decreased significantly,while the annual 90th percentile daily maximum 8-hour concentration of O₃ increased slightly.The ambient air quality of cities in Tibet negatively correlated with population and economic development.Influenced by the pollution source emissions,meteorological conditions,regional transmission and other factors,the concentration of O₃ in Tibet was relatively higher in spring and summer,while the concentration of other pollutants was higher in winter.