The invention provides a method for extracting phosphorus from sludge to produce a phosphorus-containing composite fertilizer. The problems in the prior art that the phosphorus release effect is poor, the energy consumption is high, and the phosphorus recycled material is doped with heavy metal are solved. The method comprises the steps of 1, phosphorus release, wherein an acid solution is added into a sludge solution to make phosphate released into the solution; 2, phosphorus recycling, wherein supernatant with phosphate in the step 1 is taken out, PO43- and Fe3+ are made to generate ferric phosphate sediment by adding Fe3+ under preset conditions, and water washing is carried out several times on the ferric phosphate sediment; 3, phosphate fertilizer preparation, wherein a reducing agent is added into the ferric phosphate sediment, and Fe3+ is reduced into Fe2+, accompanied by dissolving of PO43+; MgO, Mg(OH)2 and CaO or Ca(OH)2 is added to adjust the pH so as to form a raw material of the phosphorus-containing composite fertilizer, and the raw material of the phosphorus-containing composite fertilizer is processed into the phosphorus-containing composite fertilizer. According to the method for extracting phosphorus from the sludge to produce the phosphorus-containing composite fertilizer, waste is turned into wealth, resource utilization of phosphorus in the sludge is achieved, and the resource utilization rate of phosphorus in the sludge can reach 92% or above.