The invention provides an anti-freezing modular lightweight green slope roofing system and a construction method thereof. The roofing system includes a structural layer and an equipment unit. The structural layer from the bottom up includes a root barrier layer (100), a water filtering layer (200) and a planting layer (300), wherein the root barrier layer (100) is used for inhibiting plant roots from entering a building structure and plays a leakage prevention effect; the water filtering layer (200) is used for filtering irrigation water and draining the irrigation water out of a roofing structure; and the planting layer (300) is used for supporting plant growth. The equipment unit includes an irrigation anti-freezing device, so that automatic irrigation can be carried out at low temperatures during a frost period. A plurality of interconnected planting rows (400) are arranged on the root barrier layer (100) to implement modularization of the roofing system. The roofing system and theconstruction method thereof can effectively solve problems such as difficult irrigation during the frost period, soil erosion, pipeline blockage, high plant management cost and difficult repair, create positive effects, and are conducive to being popularized and used.