This paper presents a three-dimensional nonlinear biomechanical finite element model to simulate elderly breast deformation under arm abduction. The finite element model was constructed based on complex anatomical structures that consist of the soft tissues including torso, breast, pectoralis major muscle, and rigid bones including humerus, sternum, clavicle and ribs. The elderly breast was defined as material nonlinearity and geometric nonlinearity. The computational model can simulate and illustrate the deformation of soft tissues during arm abduction from 30° to 90°. The finite element model was validated by motion data. The Young’s modulus for the clavicular portion and the sternocostal portion of the pectoralis major muscle was characterized as 0.1 MPa and 0.08 MPa, respectively. Besides this, the finite element model presented here features the musculoskeletal system for breast deformation and reveals the synergistic relationship between the breast and the pectoralis major muscle. A questionnaire was conducted to analyze the importance of purchasing factors and the discomfort positions in a sports bra from the perspective of senior women. Combined with the finite element model results, this study provides a promising basis for the design of sports bras in an ergonomic way.