The optical properties of GaAsSb-capped InAs quantum dots (QDs) with different capping layer thickness are investigated. Both the emission energy and the recombination lifetime are found to be correlated with the capping layer thicknesses. Theoretical calculations indicate that the quantum confinement and the wave function distribution of hole states are sensitive to the GaAsSb capping layer thickness. The Sb induced change in QD size also plays a role in the optical properties of GaAsSb-capped QDs. Controlling the GaAsSb capping layer thickness is a feasible way to tailor the InAs QDs for long-wavelength applications.