Using the plane-wave ultrasoft pseudopotential method and generalized gradient approximation which based on the first-principles density functional theory, the bond length, energy band structure and density of states of the layer of Si/Ge were calculated. The results show that, compared with the single-crystal Si layers, Si-Ge bond length get longer and the population decrease in the layer of Si/Ge . It can be seen from the energy band structure that the band structure width of the layer of Si/Ge decreases. The optical properties calculations show that, compared with the single-crystal Si layers, the absorption band of the layer of Si/Ge get narrower, and the absorption band-edge obviously get red-shift.