The invention discloses a plant growth device for afforesting buildings. The device is provided with a crossbeam parallel to the ground; conduits perpendicular to the ground are suspended on the crossbeam at intervals and are sleeved with plant growth covers; the outer surfaces of the plant growth covers are provided with planting holes; water spraying holes are distributed on the conduits; and the setting density of the water spraying holes gradually decreases from upper ends of the conduits to lower ends of the conduits. Each plant growth cover comprises an inner layer and an outer layer, agrowth matrix is arranged between the inner layer and the outer layer, and the inner layer is provided with water suction strips. The device realizes the three-dimensional plantation of plants, occupies smaller land area, and is applicable to urban afforestation. After the plants are planted, the device is just like a curtain wall formed by the plants, and can effectively improve urban environment, beautify urban environment, and perfect urban ecological balance. Nutrient solution is uniformly sprayed onto the plant growth covers through the water spraying holes of the conduits; and the innerlayer of each plant growth cover is provided with the water suction strips, so that the water retention property can be strengthened and the growth of the plants is facilitated.