In the present paper, the non-equilibrium grain-boundary segregation of P atom was studied in low alloy steels subjected to a low tensile stress at different temperatures. The AES (Auger electron spectroscopy) experiments and dynamic analyses were conducted to study on the non-equilibrium grain-boundary segregation of P atom. The research results show that non-equilibrium segregation of phosphorus occurred at the grain boundaries of the steels 2.25Cr1Mo and 12Cr1MoV, while the critical time reached about 0-1 hour at constant temperatures 773 and 813K. The relationship between the diffusion rate and the diffusion time for the complex and the phosphorus atom was investigated based on the experimental results. Eventually the diffusion coefficients of complex and P were calculated with using a proposed dynamic model.