The invention discloses a flat roof stagnant water greening device. The flat roof stagnant water greening device comprises a stagnant water box, a greening planting box and a purification circulation system; the greening plant planting box is detachably connected with the top of the stagnant water box through a connecting piece; the stagnant water box comprises a square frame body consisting of a plurality of vertically arranged supporting rods and a plurality of connecting rods connected among the supporting rods; the bottom of each supporting rod is connected with a telescopic supporting foot; sealing plates are arranged at the bottom and on the sides of the square frame body; an overflowing port is formed in one sealing plate on the sides; the greening planting box comprises a box body and a plurality of supporting legs connected with the bottom of the box body; a plurality of planting areas are distributed in the box body in a grid form; a plurality of evaporation holes are distributed at the bottom of the box body uniformly; the purification circulation system comprises a water storage pool; the water storage pool is communicated with the overflowing port of the stagnant water box through a purification pipeline; and the water storage pool is connected with a booster pump. Compared with the prior art, the flat roof stagnant water greening device has the advantages that the utilization rate of rainwater is increased and the city heat island effect is reduced.