Abstract: The material of the secondary guide vanes of a high-pressure turbine in a certain aircraft engine was DD6 single crystal high-temperature alloy. After testing, it was found that one of the secondary guide vane basins cracked at the lower edge plate near the inlet edge. The causes of guide vane cracking were analyzed using macroscopic observation, scanning electron microscopy, energy spectrum analysis, metallographic examination, and other methods. The results show that there were casting defects in the crack source area of the lower edge plate of the high-pressure turbine secondary guide vane. The casting defects were impurities formed during the blade casting process, and the orientation difference between the impurities and the edge plate matrix grains was 41.9°. The crack on the lower edge plate of the second stage guide vane of the high-pressure turbine was a thermal fatigue crack, and impurity defects were the main cause of early thermal fatigue cracking of the guide vane.