Optical properties and surface structures of InAs/GaAs serf-assembled QDs grown on 2ML GaSb and x-ML GaAs combined strain-buffer layer are investigated systematically by photoluminescence (PL) and atomic force microscopy (AFM). The QD density varies from 1.2×10cm-2 to 8.0×10cm-2 due to the influence of the lattice mismatch. The combined layer favors the increasing of In incorporated into dots and the average height-to-width ratio，which resulted in the red-shift of the emission peaks.For the sample of 5ML GaAs thin film the ground state transition is shifted to nearly 1300nm at room temperature.