This paper describes the behavior of phosphorus during the gasification of dewatered sewage sludge in supercritical water with an alkaline additive. Dewatered sewage sludge was treated in an autoclave at 400 degrees C for 30 min. Without additives, phosphorus is enriched in a solid residue. The phosphorus transferred from the solid residue to the liquid product, and the phosphorus content in the liquid product increased from 41.0 to 2214.5 mg/L in the presence of 2-8 wt % alkaline additives. The pathway of phosphorus under the catalytic condition is proposed. Analysis of the distribution of phosphorus forms in the solid residue, and XRD results indicate that the alkaline additive combines with solid phase phosphate to form analcime and kalsilite, which releases phosphate into the liquid phase. The concentration of phosphorus in the liquid product of supercritical water gasification with the alkaline additive was close to that obtained by chemical extraction.