The invention relates to a water-harvesting type roof rainwater garden. The water-harvesting type roof rainwater garden comprises an outdoor greening system and a rainwater circulating system, wherein the outdoor greening system comprises a roof vegetation layer, a sandy soil layer, a grit layer, a geotechnical cloth layer, a crushed stone cushion layer, a gravel layer, a barrier layer, a secondary filter layer and a rammed earth layer from top to bottom; the rainwater circulating system comprises a water storage tank, a water purifier, a water pump, a first electromagnetic valve, a second electromagnetic valve, a third electromagnetic valve, a soil humidity sensor, a first pipeline, a second pipeline, a third pipeline, a fourth pipeline and a PLC (programmable logic controller). According to the water-harvesting type roof rainwater garden, the problem about drainage and filtration of rainwater are solved, a graceful landscape environment space is created, and the rainwater is collected, purified and recycled perfectly.