Green roofs have good heat preservation and insulation performance, which plays an important role in reducing air-conditioning load and reducing carbon dioxide emissions. Experimental research and numerical simulation analysis on the energy-savings and ecological benefits of green roofs in hot summer and cold winter areas in Wuxi, China (31°N, 120°E) are carried out. The experimental results show that a new type of Buddha grass can absorb 1.79 kg of CO2 and release 1.3 kg of O2 per square meter per year. In addition, the annual carbon emission reduction of the green roof in Wuxi was 9.35 kg m–2, and the emission reduction benefit of the green roof was calculated to be USD $1.02 m–2 a–1. The energy consumption simulation shows that the green roof per unit area can save 11.53 kWh per year, and the annual quantitative ecological benefit of a green roof is USD $3.37 m–2. The calculated dynamic investment return is 10 years. A green roof has the benefits of heat preservation and insulation; meanwhile, it can reduce energy consumption. It is also worth mentioning that its energy-saving potential and ecological benefits are substantial, and the investment payback time is short, which makes it worth promoting in a large area.