The invention discloses a powder spraying top lance for RH vacuum refining. The powder spraying top lance comprises a laval tube, a core tube, a top lance center tube, a top lance power conveying tube, a top lance inner tube, a top lance outer tube and a top lance waterproof tube, wherein an annular shrinkage tube section and an annular expansion tube section are formed between the laval tube and the core tube. The annular shrinkage tube section and the annular expansion tube section are capable of accelerating airflow to supersonic speed, thereby improving the working lance height, reducing splash steel liquid adhered to the lance and prolonging the service life of a nozzle of the top lance; meanwhile, a throat between the laval tube and the core tube is an annular throat, so that the oxygen blowing flow can be increased, and the heating efficiency of oxygen-blowing decarbonization and oxygen-blowing aluminum addition can be improved; oxygen, gas and desulphurization powder are conveyed to an RH vacuum chamber through different channels; the desulphurization powder can reach the surface of the steel liquid under the surrounding of ambient protective gas; the powder taken away in a vacuum pumping manner is reduced, and the yield of the desulphurization powder is improved; moreover, compared with the existing top lance, the powder spraying top lance for RH vacuum refining is free of precision casting, so that the manufacturing difficulty is obviously reduced.