Abstract: M2 high speed steel was cast with different size mould. The effect of cooling rate change on the microstructure of high speed steel, especially the morphology and structure of eutectic carbide was studied. The results show that when the diameter of the ingot is reduced, the cooling rate is accelerated, and the carbide distribution of high-speed steel is more uniform and the shape of M₂C carbide changes from flake to fiber. Compared with flaky carbides, fibrous carbides are less stable and easier to decompose by heating, which makes carbides spheroidized and refine obviously, improve the microstructure uniformity of high-speed steel, and it is beneficial to obtain good hardness and toughness matching.