We developed a novel indicator, daily excessive concentration hours (DECH), to explore the acute mortality impacts of ambient fine particulate matter pollution (PM) in Hong Kong. The DECH of PM was calculated as daily concentration-hours >25 μg/m. We applied a generalized additive models to quantify the association between DECH and mortality with adjustment for potential confounders. The results showed that the DECH was significantly associated with mortality. The excess mortality risk for an interquartile range (565 μg/m*hours) increase in DECH of PM was 1.65% (95% CI: 1.05%, 2.26%) for all natural mortality at lag 02 day, 2.01% (95% CI: 0.82%, 3.21%) for cardiovascular mortality at lag 03 days, and 1.41% (95% CI: 0.34%, 2.49%) for respiratory mortality at lag 2 day. The associations remained consistent after adjustment for gaseous air pollutants (daily mean concentration of SO, NO and O) and in alternative model specifications. When compared to the mortality burden of daily mean PM, DECH was found to be a relatively conservative indicator. This study adds to the evidence by showing that daily excessive concentration hours of PM might be a new predictor of mortality in Hong Kong.