Abstract: The low pressure cast AlSi7Cu aluminum alloy specimens for automobiles were produced by using different coating thicknesses and sand types, and the length of the dendrite arm of the microstructure was measured.It was also used to evaluate the effect of the solidification speed and the cooling speed on the alloy microstructure.The results are that in the process of low pressure casting of aluminum alloys, with the increase of the thickness of the mold thermal insulation coating, the lower the maximum temperature of the casting mold can be, and the greater the temperature of the casting is.Through the microstructure observation of low-pressure metal casting and sand casting and the measurement of the secondary dendrite arm spacing, it is found that with the increase of the thickness of the metal coating, the secondary dendrite arm spacing on the edge and heart of the casting is gradually increased.The greater the coating thickness, the lower the casting cooling speed.From the edge to the heart, the secondary dendrite arm of the sand cast samples spacing changes from small to large.The secondary dendrite arm spacing of the organic sand sample is greater than that of the inorganic sand sample, inorganic sand has faster heat transfer and has a greater cooling rate.