The surface quality of the substrate has an important effect on the quality of the growth film. The roughness, uniformity, adhesion residue and oxide layer of the substrate are the evaluation criteria of its surface performance. In this paper, an in-situ thermal treatment method to remove the natural oxides on GaSb (001) substrates for epitaxial growth of CdZnTe films by close-spaced sublimation method was reported. By controlling the temperature and time of the thermal treatment, a clean and smooth substrate state is obtained. The effect of thermal treatment on the morphology and composition of GaSb substrate was analyzed by atomic force microscopy and X-ray photoelectron spectroscopy. The crystal quality of CdZnTe epitaxial film grown on GaSb substrate after thermal treatment was evaluated by double crystal X-ray curve. In order to further study the properties and epitaxial formation mechanism of the micro-defects near the heterogeneous interface, TEM analysis of CdZnTe/GaSb cross section was also carried out. After 180 s thermal treatment at 600 ℃, the GaSb substrate can obtain a clean and relatively flat surface after most of the oxide is removed from the substrate surface, thus improving the crystallization quality of CdZnTe epitaxial film. The full width of half maximum of double crystal X-ray curve is 94″, which approached the crystalline quality of bulk CdZnTe crystal ever reported.