Abstract: The sludge of sewage treatment plants is rich in phosphorus and recycling phosphorus from sludge can alleviate phosphorus depletion to a certain extent. Sludge was treated by incineration, with rice husk as an additive, and the variation of phosphorus fraction and extraction efficiency in sludge-rice husk incineration ash were investigated by using Hedley's stepwise extraction method and acid extraction method. The effect of rice husk dose on phosphorus fraction in incineration ash was explored. The results showed that the phosphorus bioavailability was the best when the dose of rice husk was 50% of the total mass. Based on the optimization by response surface method, the optimal condition was found that HCl concentration, extraction time, and the ratio of liquid and solid were 0.33 mol/L, 6.4 h, and 50 mL/g, respectively. The phosphorus extraction efficiency at the optimal experimental condition was as high as 93%, which was 20% higher than that of sludge incineration ash. Through comparing the physical and chemical properties of sludge ash, rice husk ash and ash of co-incineration, it was found that the co-incineration of sludge and rice husk did not change the main components of sludge ash and rice husk ash, but the new phase was generated and the morphology changed. The results illustrated that the co-incineration of sludge and rice husk could improve the phosphorus extraction efficiency of incineration ashes, which was beneficial to the recovery of phosphorus resources from sludge.