Abstract: When 718 steel ingot was used to process module, it was found that the flaw detection does not meet the requirements of class III in GB/T 6402-2008 standard. The causes of defects have been analyzed by means of chemical composition analysis, macroscopic examination, metallographic examination and fracture analysis. The results show that the unqualified detection of module was caused by the internal flake defect. Although hydrogen content in 718 steel ingot was low, rapid cooling and no dehydrogenation annealing process after processing and manufacturing of module led to flake defect in module. By improving the quality of the base material, adding the process of slow cooling and dehydrogenation annealing, the flake defect would be prevented.