For most of the experiments and the practical problems about the non-equilibrium grain-boundary segregation (NGS), segregation will always occur during continuous cooling. The effective-time-method assumed that any continuous cooling curve for a sample can be replaced by a corresponding stepped curve. In this paper, the non-equilibrium grain-boundary segregation of phosphorus in step cooling process in an industrial steel12Cr1MoVis studied based on the effective-time-method and compared with that in isothermal holding process. Results show that the calculated result is in good accordance with the experimental observation. It provides a direct support for the effective-time-method of NGS. The step cooling process can speed-up the embrittlement of the steel. Based on the step cooling technique, a control method of grain-boundary brittleness is proposed.