The preference site and magnetic property of doped Cr atom in L10 FePt alloy are investigated by the first-principles calculations. Due to the energy of Cr located in Fe and Pt sublattice are -0.15 and 0.48 eV, respectively, the Cr atom occupying Fe site is more energetic favorable than in Pt site in L10 FePt alloy. The calculated saturation magnetizations for Cr additive replacing Fe or Pt atom in L10 FePt are 959 or 989 emu/cm3, respectively. The results are lower than that of L10 FePt (1085emu/cm3). The additive of Cr in L10 FePt will reduce the saturation magnetization.