This paper investigates the water quality characteristics of rainwater runoff from dual-substrate-layer green roofs in Tianjin, China. The data were collected from four different assemblies and three types of simulated rains. The storm-water runoff quality was monitored from early June through late October 2012 and from July through late November 2013. The results revealed that the runoff water quality would be improved to some extent with the ageing of green roofs and that the quality retention rate better reflected the pollutant retention capacity of the green roof than the pollutant concentration in the runoff water. The investigation clearly demonstrated that green roofs also effectively reduced the chemical oxygen demand and turbidity value and neutralised acid rain to stabilise the pH of the runoff.