The non-equilibrium grain-boundary segregation of phosphorus in step cooling process in an industrial steel,12CrlMoV, is studied based on the effective-time-method and compared with that in isothermal holding process. The non-equilibrium grain-boundary segregation concentration of phosphorus was measured with Auger Electron Spectroscopy(AES) and calculated based on the kinetic equations of non-equilibrium grain-boundary segregation. Results show that the calculated result is in good accordance with the experimental observation.