Abstract: After using the 30CrMnSiNi2A ultra-high strength steel aircraft front landing gear mounting shaft for a period of time, multiple cracks were found in the circumferential direction of the R angle near the end support surface of the mounting shaft through magnetic particle inspection. The reasons for the cracking of the mounting shaft were analyzed using macroscopic observation, scanning electron microscopy and energy spectrum analysis, metallographic examination, hardness testing, chemical composition analysis. The results show that the cracking property of the mounting shaft was brittle cracking. After phosphating and chrome plating treatment on the surface corner area of the R corner root of the mounting shaft, early hydrogen embrittlement cracks were generated. After major repairs and installation, the stress state of the mounting shaft changed, and the cracks experienced fatigue propagation, ultimately led to the mounting shaft cracking.