Background: Air pollution in Beijing has been improving through implementation of the Air Pollution Prevention and Control Action Plan (2013–17), but its implications for respiratory morbidity have not been directly investigated. We aimed to assess the potential effects of air-quality improvements on respiratory health by investigating the number of cases of acute exacerbations of chronic obstructive pulmonary disease (COPD) advanced by air pollution each year. Methods: Daily city-wide concentrations of PM10, PM2·5, PMcoarse (particulate matter >2·5–10 μm diameter), nitrogen dioxide (NO2), sulphur dioxide (SO2), carbon monoxide (CO), and ozone (O3) in 2013–17 were averaged from 35 monitoring stations across Beijing. A generalised additive Poisson time-series model was applied to estimate the relative risks (RRs) and 95% CIs for hospitalisation for acute exacerbation of COPD associated with pollutant concentrations. Findings: From Jan 18, 2013, to Dec 31, 2017, 161 613 hospitalisations for acute exacerbation of COPD were recorded. Mean ambient concentrations of SO2 decreased by 68% and PM2·5 decreased by 33% over this 5-year period. For each IQR increase in pollutant concentration, RRs for same-day hospitalisation for acute exacerbation of COPD were 1·029 (95% CI 1·023–1·035) for PM10, 1·028 (1·021–1·034) for PM2·5, 1·018 (1·013–1·022) for PMcoarse, 1·036 (1·028–1·044) for NO2, 1·019 (1·013–1·024) for SO2, 1·024 (1·018–1·029) for CO, and 1·027 (1·010–1·044) for O3 in the warm season (May to October). Women and patients aged 65 years or older were more susceptible to the effects of these pollutants on hospitalisation risk than were men and patients younger than 65 years. In 2013, there were 12 679 acute exacerbations of COPD cases that were advanced by PM2·5 pollution above the expected number of cases if daily PM2·5 concentrations had not exceeded the WHO target (25 μg/m3), whereas the respective figure in 2017 was 7377 cases. Interpretation: Despite improvement in overall air quality, increased acute air pollution episodes were significantly associated with increased hospitalisations for acute exacerbations of COPD in Beijing. Stringent air pollution control policies are important and effective for reducing COPD morbidity, and long-term multidimensional policies to safeguard public health are indicated. Funding: UK Medical Research Council.