Doping the third element is an important method to promote ordering transition of FePt and reduce the ordering transition temperature. However, it is a great challenge to explore the mechanism of the third element promoting FePt ordering and clarify the role of the third element in FePt ordering transition. Therefore, this work has investigated the effects of Bi element on the lattice constant, formation energy, differential charge density, saturation magnetization and ordering transition temperature of FePt with the first principle density functional theory. The results show that the substitution formation energy of Bi atom replacing Fe atom is lower than that of Pt atom. Bi atom is easier to replace Fe atom. The substitution formation energy of Bi atoms replacing near neighbors is lower than that of far neighbors. Bi atoms tend to aggregate in the lattice. Fe atoms are the main source of magnetism of FePt system. Bi atoms have little effect on the electronic structure and magnetism of FePt. The ordering transition temperature of FePt can obviously be reduced after doping Bi element. In the case of double doping, the lowest ordering transition temperature of FePt system is 623.32 K. The vacancy mechanism plays a leading role in the ordering transition. Bi element reduces the vacancy formation energy of FePt, increases the vacancy concentration of Fe and Pt atoms in the system, and promotes the diffusion and migration of Fe and Pt atoms, so as to promote the ordering transition of FePt.