Abstract: A physical and a mathematic model of solute grain boundary segregation were developed which synthetically took account of both equilibrium and non-equilibrium grain boundary segregation, transformation of segregated solute to precipitates and cooling rate. The adsorption and adsorptivity of grain boundaries and extended disordered zones in the vicinity of grain boundaries to solute atoms were considered. The evolution of boron grain boundary segregation during cooling from 1150℃ to 640℃ in the Fe-40%Ni-B alloy with 0.0010% boron was simulated. The simulated results show that the boron enrichment factor of the boundary zones increases fast at the beginning of cooling, then relatively even, indicating the back diffusion of segregated boron atoms. After boron precipitates appearing, the enrichment factor increases fast again. Good agreement between experimental and simulated results was achieved.