A systematic investigation is given about the influence of thickness of the spacing layer on the strain distribution during the capping stage of the quantum dot. The calculated results show that the mismatch between the lattice constant of the spacing layer along the growth direction and that of the quantum dot is very important for the the strain distribution. The height of the quantum dot is compressed during the capping stage, which is qualitatively interpreted form the strain distributions. When the thickness of the spacing layer equals the quantum dot, the influence of thickness of the capping layer on strain distribution is also discussed. Based on the deformation potential theory, the dependence of the emission wavelength on the thickness of the capping layer is investigated. The calculated results agree well with the experiment results. We conclude that, during the capping stage of the quantum dot, the strain is very critical for both the shape of the quantum dot and the optical characteristics; extension of the emission wavelength via quantum dot strain engineering is an effective means.