Optical properties and surface structures of InAs/GaAs selfassembled quantum dots(QDs) grown on 2 nm In_(0.2)Ga_(0.8)As and x ML GaAs combined strain-buffer layer were investigated systematically by photoluminescence(PL) and atomic force microscopy(AFM).The QD density increased from ~1.7×10~9 cm~(-2) to ~3.8×10~9 cm~(-2) due to the decreasing of the lattice mismatch.The combined layer was of benefit to increasing In incorporated into dots and the average height-to-width ratios,which resulted in the red-shift of the emission peaks.For the sample of x= 10 ML,the ground state transition is shifted to 1350 nm at room temperature.