The invention discloses a watering-fertilizing integration water retention agent, a preparation method thereof and an application thereof. The water retention agent is prepared from a water soluble polymerization monomer, bagasse or a bagasse derivative, ground phosphate rock, a free radical polymerization initiator, and a crosslinking agent. The method of the invention, which treats bagasse as araw material, allows an agricultural waste resource to be effectively utilized, and the cheap and abundant ground phosphate rock is added in the method, so the cost of a product (the water retention agent) of the invention is low. In addition, the ground phosphate rock, which is also used as a raw material in the method of the invention, can be reacted with residual acidic monomers and can release water-soluble phosphor which can be absorbed by plants, so the poisoning effect brought by the residual acidic monomers is substantially reduced; and the ground phosphate rock can stably and durablymaintain and release nutrients needed by the growth of crops through a swelling effect under the effect of a sustained and controlled release system of the water-retaining agent, so the water retention agent allows soil to be more suitable for the plants to grow and the biological utilization degree of the nutrients in a fertilizer to be improved.