[Objective] To explore the best lateral positioning angle to prevent deep pressure injury by a mechanical analysis with the three-dimensional finite element sacrococcygeal model at different lateral positioning angles. [Methods] The sacrococcygeal CT data of a healthy adult male volunteers in prone position were used to construct a three-dimensional finite element model, including skin adipose tis- sue, muscle tissue and bone at 0°, 15°, 30° and 45° lateral positioning angles. As boundary conditions were set and loads were applied, the Von Mises stress, shear stress and distribution characteristics of pressure to damage the sacrococcygeal soft tissues were measured. [Results] The Von Mises stress on the skin and adipose tissue were ranked in ascending order of 30° group ＜15° group ＜0° group ＜45 ° group with a statistically significant difference among them [(5.1±0.1) kPa, (5.6±0.2) kPa, (6.1 ± 0.1) kPa, (12.7±1.2) kPa, P＜0.001] , whereas the Von Mises stress on muscle tissue were down-up in order of 30° group ＜45° group ＜15° group ＜0° group, which was statistically significant [(17.6±0.8) kPa, (19.3±1.4) kPa, (20.6±0.5) kPa, (22.6±1.4) kPa, P＜0.001] . In addition, the shear stress on the skin and adipose tissue was in acending order as 30° group ＜15° group ＜0° group ＜45 ° group, which was statistically significant [(0.8±0.2) kPa, (0.9±0.1) kPa, (1.3± 0.2) kPa, (2.6±0.8) kPa, P＜0.001] , however, the shear stress on muscle tissue was ranked as 30° group ＜ 15° group ＜ 45° group ＜ 0 ° group with a statistically significant difference [(4.4±0.2) kPa, (4.4±0.2) kPa, (4.7±0.5) kPa, (5.1±0.3 ) kPa, P=0.02] . By comparison between tis- sues, both Von Mises stress and shear stress on the muscle tissue were significantly higher than those on the skin and adipose tissue under the 4 lateral positioning angles (P＜0.05) . [Conclusion] The Von Mises stress and shear stress on the sacrococcygeal soft tissue are the least at the lateral positioning angle of 30°, so it is recommended to use the lateral decubitus position at 30° to prevent the stress injury in the deep tissue.