Abstract: A 12.9 grade bolt used in the suspension of a certain model of new energy vehicle broke during dynamic testing. The causes of bolt fracture were analyzed using macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination, microhardness testing. The results show that the incomplete removal of the phosphating film and insufficient tempering temperature before heat treatment resulted in high hardness of the bolt, with a phosphorus infiltration layer on the surface and high phosphorus content, significantly reduced the toughness of the material and ultimately leading to brittle fracture of the bolt.