The invention discloses a vertical greening device for building walls, and belongs to the greening device for the building walls. The vertical greening device comprises a steel wire net piece, a waterproof hanger used for fixing the steel wire net piece, a plurality of wall-mounted flowerpots distributed on the steel wire net piece, and a water supply device. The waterproof hanger 1 comprises a glue-injected waterproof base and a hanging piece. The hanging piece is of a structure which is bent by 90 degrees and then bent by 90 degrees in a reverse mode and is provided with an installation hole. The water supply device is a water supply pipe arranged transversely along the grids of the steel wire net piece and provided with a plurality of water outlet holes. The wall-mounted flowerpots are distributed on the steel wire net piece in a matrix mode. According to the vertical greening device, an irrigation system is simple, the water outlet holes of the water supply pipe directly face the portions above the flowerpots, and overflow holes are formed in the upper portions, the middle portions and the lower portions of the flowerpots so that the problem of excessive watering can be solved. The vertical greening device is low in manufacturing cost, easy to maintain and long in service life.