Abstract: Cracks were found in the thread part of 17-4PH stainless steel high lock nut during processing. The causes of cracks in the nut were studied by microscopic observation, processing technology analysis and extrusion process analysis. The results show that the cracks were generated during the extrusion process. The hardness of the billet before extrusion was high and the extrusion deformation was too large, which made the plasticity of the material and the fluidity of the core worse, and finally led to the crack in the core of the nut. Double annealing treatment of the billet before extrusion could avoid cracks in the nut during processing.