The ordering behavior and the mechanical properties come from the doped element of the L1₂ type Al₃Sc-based intermetallics were studied by using sublattice model supported with first-principles calculations. The results show that the intermetallics Al₃Sc is fully ordered intermetallics, where Al atoms always occupy the 3c sublattice and Sc atoms always occupy the 1a sublattice. L1₂-Al₃(Sc_0.75M_0.25) intermetallics(where M=Y, Ti, Zr and Hf)is fully ordered intermetallics, where M always occupy the 1a sublattice. The site preferences of these alloying elements are independent of the heat treatment temperature. L1₂-Al₃(Sc_0.75M_0.25) intermetallics satisfy the mechanical stability conditions.The shear modulus, elastic modulus, young"s modulus and the hardness of L1₂-Al₃(Sc_0.75Y_0.25) intermetallics are are smaller than L1₂-Al₃Sc. The shear modulus, elastic modulus, young"s modulus and the hardness of L1₂-Al₃(Sc_0.75M_0.25) intermetallics, where M=Ti, Zr or Hf, decrease with the increasing of the atomic radius. L1₂-Al₃(Sc_0.75Ti_0.25) has the best plasticity and toughness compared with other L1₂-Al₃(Sc_0.75M_0.25).