Abstract: Taking 3 kinds of as-cast high chromium cast iron with Mo content as the research object, the cold and hot cycle thermal fatigue test was carried out. The effects of Mo content on the microstructure and hermal fatigue properties of the high chromium cast iron were studied with X ray diffractometer, optical microscope, scanning electron microscope, transmission electron microscope, and the thermal fatigue resistance mechanism of high chromium cast iron was discussed. The results show that the as-cast high chromium cast iron is mainly composed of α-Fe, γ-Fe, and (Cr, Fe)₇C₃ phases, with the increase of Mo content, the austenite content in the structure increases. The high chromium cast iron is crystallized in a hypoeutectic mode, and the microstructure is mainly composed of primary dendritic solid solutions (γ-Fe) and the eutectic structure (α-Fe+ (Cr, Fe)₇C₃) between the dendritic solid solutions. The addition of Mo is helpful to refine the dendritic structure and refine shorten rod eutectic carbide. At a higher fatigue test emperature, the main crack length of the high chromium cast iron sample is longer and thicker, and there are obvious secondary cracks. With the increase of Mo content, the main crack becomes shorter, the crack becomes finer, and the secondary crack decreases. By adding Mo alloying, the thermal fatigue resistance of as-cast high chromium cast iron can be increased by 30% at 900 ℃.