Under the complex loads, the injector guide pillar (IGP) used in injection machine was failure only after 1.5-year-service. In order to determine the cause of the fracture, the numerical simulation technology was applied to analyze the mechanical properties of the IGP. The contact between the mating surfaces of the clamping mechanism was modeled; nonlinear multi-region contact of surface-surface was applied to establish the contact model of FEA. The constraint of tie was used for modeling thread joint. The simulated results indicated that the smaller area of contact surface, the higher value of stress in the neck of IGP. Electronic measurement was also used to check the results of stress in IGP obtained by FEA. It was found that the experimental data agreed well with simulated results. Based on the numerical analysis and experimental study, the structure of IGP was improved by adopting a smoother double-round neck. The fatigue life of the improved structure was longer than that of the original machine.