Abstract: Internal defect was found in a Zr-1 zirconium ingot during nondestructive testing. The cause of the defect was analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, hardness test, scanning electron microscope and energy spectrum analysis. The results show that the defect of zirconium ingot was caused by the contamination of the electrode used during smelting and the introduction of foreign inclusions, which led to the introduction of copper into Zr-1 zirconium ingot, the formation of zirconium copper alloy by unmelted copper oxide particles and molten copper, and the formation of hard and brittle inclusions in Zr-1 zirconium ingot.