The invention provides a roof afforestation system which comprises roof planting modules, sectional buffering modules, a drain blind pipe, a drain eave gutter and a drainage device. Each roof planting module comprises planting soil and a vegetation unit, each sectional buffering module comprises a pebble groove, soil retaining units and filter layers, the roof planting modules are formed on a roof through the sectional buffering module, each sectional buffering module is arranged between two corresponding adjacent roof planting modules, the soil retaining units are arranged on two sides, in a slope direction of the roof, of each pebble groove, and each filter layer is arranged on one side, close to the corresponding roof planting module, of the corresponding soil retaining unit; the drain blind pipe is arranged below the planting soil between the sectional buffering modules and the drain eave gutter, the drain blind pipe penetrates the filter layers and the soil retaining units and is communicated with the pebble grooves, and water on the roof flows into the drain eave gutter through the pebble grooves and the drain blind pipe and is discharged through the drainage device. The roof is divided into the roof planting modules through the sectional buffering modules, sloping length of each roof planting module is reduced, and uniformity in soil distribution and soil humidity caused by gravity erosion of rainwater is lowered.