The invention discloses a greening irrigation system for sponge cities. The greening irrigation system comprises a water collecting device, a priming pump device, an aeration device and a greening device, wherein the water collecting device comprises a water collecting pool, a water collector and a purification integrated machine; the priming pump device comprises water feed pumps connected with pump water discharge pipes, the pump water discharge pipes are connected with the aeration device which is connected with a water storage tank, and the water storage tank is connected with water storage tank discharge pipes which are connected with the greening device. The construction method comprises steps as follows: (1), excavating foundation pits; (b), building the water collecting pool, an aeration tank and the water storage tank; (c), pretreating a foundation where ecological greening insert blocks are to be laid; (d), laying the ecological greening insert blocks; (e), planting green plants. According to the system, urban rainwater is collected and treated by the water collecting device, the collected rainwater is purified, put into secondary use and applied to irrigation of the greening device, so that not only is the waterlogging phenomenon of the road surface prevented effectively, but also water resource waste can be reduced through rainwater collection.