The invention relates to a tree, flower and grass root liquid adding device and a liquid adding method thereof, and belong to the field of forestry and flowers. The liquid adding device is of a tubular structure, the lower portion of the tubular structure is of an inverted cone structure, seepage holes are formed in the periphery of the inverted cone structure, a liquid adding port is formed in the upper portion of the tubular structure, a filter disc is arranged in the liquid adding port, and a pipe cover is arranged on the liquid adding port. The device can effectively prevent oxygen deficitof tree roots and promote healthy growth of nursery stocks, a plurality of liquid adding devices can be arranged on soil for planting trees, flowers and plants, water, nutrition, liquid medicine or soil loosening agents and the like can be supplemented to the trees, the flowers and the plants, growth of the trees, the flowers and the plants is promoted, the survival rate of the transplanted trees, the flowers and the plants is increased, recovery of the growth state of the transplanted trees, flowers and plants is accelerated, diseases of the trees, the flowers and the plants are reduced or prevented, and stress resistance of the trees, the flowers and the plants is improved.