Green roofs have been reported to significantly contribute to the increase of urban green space, alleviation of heat island effect, energy saving from heat isolation by plant covers, pollution abatement and noise reduction. Moreover, the green roof’s capability to manage stormwater runoff makes it a hot topic in the current research. To explore the performance of water storage by extensive green roof, the comparison of different vegetation types in different substrates including Festuca arundinacea Shreb, Origanum onites L. and Sedum sediforme (Lacq.) Pau., coupled with 8 or 16 cm substrate depth was conducted. Two experimental periods with shifted irrigation amount of 3.6 and 6.0 mm·d^-1 were applied. It was found that green roof with Origanum or Festuca had the greatest water absorption capacity during summer, which led to great runoff reduction performance, while green roof with Sedum would lead to the poorest runoff reduction as it was not able to consume water fast enough.