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Grain boundaries are important microstructures in metallic materials, and dislocation nucleation on grain boundaries plays a crucial role in material deformation. Studies on P segregation on grain boundaries typically focus on segregation behavior at substitution or interstitial positions. Less attention is given to the interaction of a certain amount of P with dislocation nucleation and grain boundary structural units during deformation. Previous deformation mechanism studies often qualitatively determine mechanisms, struggling to quantitatively explain their roles. This paper offers an atomic mechanism for grain boundary dislocation nucleation, aiding in understanding of the effect of P segregation in BCC Fe on mechanical properties. In this paper, a large-scale atomic/molecular massively parallel simulation (LAMMPS) program was employed. It investigated the effect of P segregation in BCC Fe grain boundary on the tensile deformation behavior. Binary Fe-P embedded atom potential was used for the calculation of stress-strain curves and dislocation activation energy densities. The analysis of P segregation behavior on different grain boundary structures and the effect of temperature was conducted using visual analysis software (OVITO.3.10). To illustrate the elastic-plastic component of the strain during loading, a set of kinematic parameters based on continuum medium mechanics was employed. The atomic deformation tensor of the deformation gradient was calculated using the nearest-neighbor table approximation of the continuum medium field around each atom. Additionally, the mechanisms of dislocation nucleation and twinning were discussed for the symmetrically inclined grain boundaries in BCC Fe. For the (111) grain boundary system, dislocation nucleation extends and dominates the deformation behavior. For the (112) twin grain boundary system, tensile deformation is mainly controlled by twin grain boundary, resulting in BCC-FCC phase transitions. Tensile loads were applied to different grain boundaries at temperatures of 300 K and 600 K. The results indicate that, in the Fe-P system with P concentrations ranging from 0.05at.% to 0.2at.%, compared to enhancing the strength of (111) grain boundary systems, P segregation leads to a decrease in strength for (112) systems, but the effect is minor. The P segregation hinders dislocation nucleation on the (111) and (113) grain boundaries, while it promotes dislocation nucleation at (112) grain boundaries, and P segregation has no significant effect on dislocation nucleation at (114) grain boundaries. The degree of deformation of the E structural unit at the P segregated site on the (111) grain boundary is small, and the dislocations are more inclined to nucleate in the unsubstituted structural unit near the P atom.