Objective To investigate the biomechanical effects of the medial protrusio technique on the acetabular cup in adult patients with developmental hip (DDH) after total hip arthroplasty. Methods The CT scanning data of bilateral hips from an adult patient with unilateral DDH were obtained further to develop a finite element model of the affected hemipelvis. The medial protrusio technique with various levels of medial protrusio was simulated, and the biomechanical differences between the medial protrusio and non-protrusio groups were evaluated. Results In the simulated pull-out test, the maximum anti-pull-out load strength of the non-protrusio group was 1 166 N. Compared with the non-protrusio group, the anti-pull-out load strength of the 4 mm and 8 mm medial protrusio groups increased by 45.8% and 57.1%, respectively. The peak micromotion at the cup-bone interface for the non-protrusio group was 166.4 μm in the standing phase of the gait cycle, and that of the 4 mm and 8 mm medial protrusio groups was decreased by 46.2% and 62.1%, respectively. Regarding the immediate stress distributions of periacetabular bone tissues following cup implantation, the differences between the groups were not significant. Under the loading condition of the standing phase, the non-protrusio group yielded the lowest average and peak stresses. The average stress increased with the level of medial protrusio, and the highest peak stress was observed in the 4 mm medial protrusio group. Conclusions The medial protrusio technique can improve the initial stability of the acetabular cup, and the initial stability is positively proportional to the protrusio level. However, owing to the concentration of marginal stress at the cup-bone interface, a minor medial protrusio cup with insufficient bone coverage might increase the risk of various prosthesis-related complications.