The influence of La on the Cu precipitation in bcc-Fe is determined by first-principles density functional calculations. The binding energies of La-vacancy, Cu-vacancy pairs, and La-Cu pair are calculated, and the effects of La atoms on the diffusion activation energy of Cu atoms in bcc-Fe are considered. It is found that there exist strong attractive interactions between La atom and vacancy and between La atom and adjacent Cu atom. In addition, the formation energy of the vacancy adjacent to Cu atom increases significantly with the La addition, suggesting that vacancy is difficult to form in the La and Cu segregation zone. Meantime, we find that the migration energy of Cu atom is enhanced due to the attractions of La atom to adjacent vacancy and Cu atom. The calculated results of the vacancy formation energy and migration energy indicate that Cu atom possesses a higher diffusion activation energy after the La addition, and in turn, delays the segregation and precipitation process of the Cu rich phase in bcc-Fe.