Scoliosis, the three-dimensional (3D) deformation of the spinal column, is likely to worsen without early intervention. The Boston brace has been widely used to correct abnormal spinal curvature and prevent deformity progression. However, two-dimensional (2D) X-ray images and an experienced therapist are required for a proper Boston brace to be fabricated. There has been little research on how to determine the pad positions of a Boston brace to yield a better corrective effect on a scoliotic spine. This study uses the finite element (FE) method to determine the optimal pad positions for a Boston brace. The software ANSYS 11.0 is employed to establish an FE model of the trunk and brace from S-shaped scoliosis. After the FE model is validated, fifteen FE models with pad positions rotated in a range of 0 to 30 degrees and translated in a range of 0 to 30 mm are created to investigate the effects on correcting a scoliotic spine. A 3D FE model of a scoliotic spine is established to calculate the corrective effect of a Boston brace. The FE analysis indicates that the brace pad at the rib of the apex vertebra should be rotated 20° posteriorly for the optimal corrective effect. Compared to a traditional brace, the modified brace increases the corrective effect on the thoracic and lumbar spine, and lowers the mean contact pressure of the thoracic pad. Additionally, the comfort of the modified brace is comparable to that of a traditional brace, as measured using the visual analogue scale.