Background: Increasing studies reported increased mortality risk associated with exposure to lower-than-standard air pollution. This study aimed to investigate mortality burden and life expectancy loss due to lower-than-standard particulate matter air pollution in Hong Kong, China. Methods: We obtained 17-year (2000–2016) daily time-series data on mortality, particulate matters with aerodynamic diameters <2.5 μm (PM2.5) and <10 μm (PM10). We applied a distributed lag non-linear model to fit the association of years of life lost (YLL) with PM2.5 and PM10. Total YLL attributable to PM2.5 and PM10 concentrations below ambient air quality standards were estimated. Results: We estimated 5.2% of total YLL due to PM2.5, of which, 33.5%, 52.8% and 94.5% occurred in concentrations not exceeding the criterion of WHO, the USA, and China, corresponding to 0.3, 0.4 and 0.7 years loss in life expectancy, respectively. There was 6.9% (annual 34434.4 YLL) of total YLL due to PM10, of which, 50.7%, 50.7%, 99.5% and 99.5% occurred in concentrations below the criterion of WHO, Europe, the USA, and China, which corresponded to 0.5, 0.5, 1 and 1 year loss in life expectancy, respectively. Assuming that PM2.5 and PM10 concentrations had met stricter standards, more gains in life expectancy could be expected in women than in men, in people aged up to 64 years than those of 65 years or older, and in those with respiratory disease than those with cardiovascular disease. Conclusions: Exposure to lower-than-standard particulate matters air pollution could shorten life expectancy and incur massive mortality burden in Hong Kong.