Abstract: In this paper, the linear indication detected on the magnetic particle testing of 17-4PH martensitic precipitation-hardening stainless steel and the microstructure of indication is analyzed by metallography testing method, and the indication cause is finally made sure.It is found that the linear indication detected in the magnetic particle testing process is mainly caused by acerate δ-Ferrite. The conclusion supplies foundation for indication determinant in the magnetic particle testing process of martensitic precipitation-hardening stainless steel to aviation manufacture engineering.