This study focused on the effects of external conditions on the performance of grass swales installed to improve stormwater runoff quality. Four grass swales in Beijing and Shenzhen with different underlying surface characteristics, antecedent dry weather period (ADWP), transport within the grass swales, and rainfall characteristics, were compared during 16 rainfall events in Beijing and six in Shenzhen, and were evaluated over two years. The results revealed that the impervious fraction of the catchment's surface below the swales influenced inlet pollutant concentrations, resulting in variations in their efficiency. ADWP, transport within the swales, and rainfall characteristics all had important effects on their efficiency in removing pollutants. With high pollution loads, long ADWP, and concentrated influent, the swales reduced pollutant concentrations significantly, particularly suspended solids.