Objective To design a custom 3D printed monoblock acetabular implant for reconstructing Paprosky ⅢA acetabular bone defects and to analyze the stress distribution, displacement, and clinical reliability of the implant and surrounding bone using finite element analysis (FEA). Methods Bilateral hip computed tomography (CT) data of a patient with Paprosky ⅢA acetabular bone defects were collected. Models were developed and analyzed using Mimics Medical 21.0, Geomagic Wrap 2021, Solidworks 2023, and ANSYS Workbench 2022 R1 softwares. The biomechanical performance of the custom 3D printed monoblock acetabular implant was simulated under a single-leg stance condition. Results The peak von Mises stress of the hip components was observed at the femoral stem, measuring 67.318 MPa. For the custom 3D printeded monoblock acetabular implant, the peak stress was located at the anterosuperior contact area between the implant and acetabular bone, measuring 6.935 MPa. The femoral stem exhibited a peak stress of 67.318 MPa at its junction with the femoral head. The liner’s peak stress was 1.333 MPa near the fixation of screw 9 at the superior part of the acetabular cup. The screws showed a peak stress of 2.215 MPa at the junction with the implant. For the cortical bone, the peak stress was 9.844 MPa at the distal femur, while the cancellous bone exhibited a peak stress of 0.701 MPa at its distal connection with the femoral stem. The pelvic bone’s peak stress was 8.002 MPa at the anterior transition zone between the normal acetabulum and the defect. The peak micromotion of the custom 3D printed monoblock acetabular implant at its posterosuperior area, measuring 0.114 mm. The femoral stem and head exhibited a peak micromotion of 0.132 mm at the contact interface with the acetabular liner. The micromotion range at the implant-acetabular bone interface was 0.098 mm to 0.131 mm. Conclusion Under a simulated single-leg stance condition, the stress distribution in all components and the acetabular bone surface remains below their respective yield strengths. The micromotion threshold between the acetabular cup and acetabular bone is within acceptable limits. Biomechanical analysis indicates that the patient can perform early weight-bearing rehabilitation postoperatively. However, walking or jogging rehabilitation should be approached with caution.