Objective To investigate the effect of orthosis design parameters on correction of scoliosis and orthosis-trunk interface pressure. Methods A finite element model of scoliosis was constructed to simulate the assembly effect of the orthosis. The orthosis was divided into four loading areas (left rib, right rib, anterior-left and posterior-right area) to simulate six modification conditions. In Models 1, 2 and 3, a fixed modification of 20 mm was applied on the anterior left and posterior right areas, while the displacement loads of 20, 25 and 30 mm were applied on both the left rib and right rib areas. In Models 4, 5 and 6, a fixed modification of 25 mm was applied on left rib and right rib areas, with the displacement loads of 15, 20 and 25 mm applied on both anterior left and posterior right areas. The Cobb angle, apical vertebral rotation (AVR) and interface pressure were calculated. Results The correction of Cobb angle in Models 1, 2 and 3 was 8.94°, 15.62° and 17.91°, respectively, with AVR correction of 7.53°, 6.69° and 5.87°, respectively. In Models 4, 5 and 6, the correction of Cobb angle were 14.55°, 15.62° and 16.09°, with AVR correction of 5.25°, 6.69° and 8.63°, respectively. In Model 6, the correction rate of Cobb angle and AVR were 45.48% and 41.22%, respectively, with a maximum pressure of 26.51 kPa on orthosis-trunk interface, achieving the most significant outcome. Conclusions The modification of orthosis has a significant effect on the correction of Cobb and AVR angles. The loading on the left rib and right rib areas mainly affect the Cobb angle, while the loading on anterior left and posterior right areas mainly affect the spinal axial-rotation. A modification of 25 mm on all loading areas achieves the optimal spinal correction. This study provides the quantitative data for orthosis design.