With the first-principles method of density functional theory (DFT), the structure, electronic and optical properties of wurtzite AlN supercells were obtained by substituting Ge atoms for Al and N atoms, respectively. It shows that the structure changes obviously with doping. The lattice constants, volume and bond length along c-axis of AlN increase in both doping methods, and the lattice constants change satisfies Vegard's theorem. Forming energy of Ge atom substituted for Al atom is 5.41 eV, and that of Ge atom substituted for N atom is 5.58 eV. Stability of the doped systems is lower than that of pure AlN. Magnetism is not introduced in both doping methods, but the former introduces donor impurity band and an impurity band enters valence band, which makes it becomes negative energy gap metal. The latter introduces acceptant impurity band with a band gap of 0.910 eV. Both gaps are much smaller than intrinsic AlN band gap of 4.040 eV. Conductivity of the former is significantly improved, while conductivity of the latter may be improved. It is found that imaginary part of the complex refractive index function after doping in two ways is approximately no longer to 0 in the low energy region, indicating that the absorption ability of AlN materials to low frequency electromagnetic wave is enhanced in both doping ways. Imaginary part of dielectric function of the former has a new peak in low energy region, while the latter has no obvious peak in that region. The former has a stronger absorption capacity of long wave. The energy loss is reduced in both.