The invention discloses a composite dephosphorizing agent for treating acidic wastewater containing phosphorus and a preparation method and an application method thereof. The lattice-shaped tobermorite composite dephosphorizing agent is prepared by taking potash feldspar and bauxite as raw materials and unslaked lime as an adjuvant, leading the raw materials to react with the adjuvant under the conditions of medium pressure, high temperature and hot water, and adding a little of alumina and ferric oxide powder for performing the activating treatment. In the composite dephosphorizing agent, the aluminum content is 4.0-8.0%, the iron content is 0.5-2.5%, the calcium content is 20-35% and the silicon dioxide content is 35-55%; tobermorite composite crystalloid prepared by combining the materials according to the proportioning is added into the wastewater containing phosphorus with the concentration of 100-2000 mg/L to be stirred for 5-10 min, the adding mount thereof is 0.5-3.0%, and effluent can reach the national primary emission standard; and the dephosphorization ratio is more than or equal to 99.9%. The composite dephosphorizing agent is suitable for industrial acidic wastewatercontaining the phosphorus the pH value of which is less than or equal to 7.0; after the composite dephosphorizing agent is added, no alkali is needed for regulating; after dephosphorization, the pH value of the effluent is 7.0; and no sludge is produced and sediment crystal can be recycled for more than 15 times.