The invention relates to a composite water retention agent capable of promoting plant growth, and preparation and application thereof, belonging to the technical field of water retention agents. The preparation method comprises the following steps: neutralizing a certain amount of acrylic acid in a device with mechanical stirring, respectively adding acrylamide, a crosslinking agent, bentonite and an initiator, uniformly mixing, heating the system to 35 DEG C, adding gelatin, heating for decomposition, introducing humic acid while continuously stirring until the humic acid is completely dissolved to obtain a black mixed solution with certain viscosity, and polymerizing at 55-70 DEG C in a closed static state; and drying the polymerization gel product, pulverizing, screening and the like to obtain the final product. The water absorptivity is up to 700-1000 times. The test proves that the water retention agent can effectively promote plant growth.