Objective To evaluate the mechanical behavior of the internal soft tissues of the elbow at different healing stages of a posterior capsular injury. Methods A finite element model of the elbow joint considering muscle activation behavior was established to simulate elbow flexion without injury and at 2nd, 4th, 6th, and 8th week after posterior capsule injury. The von Mises effective stress variations in the articular capsule, ulnar cartilage, and ligaments were analyzed. Results Before injury and at 2nd, 4th, 6th, and 8th week after injury, the stress on the articular capsule at 60° of flexion was 8.23, 7.87, 8.27, 8.99, and 10.5 MPa, respectively. When the elbow flexion angle was 30°, the ulnar cartilage stress increased by 13.0%, 28.3%, 41.3%, and 45.7% at 2nd, 4th, 6th, and 8th week after injury, respectively, compared with that before injury. At the 4th week after injury, the stress of the radial collateral ligament at 15°, 30°, 45°, 60°, and 75° flexion was reduced by 12.5%, 22.2%, 13.6%, 3.2%, and 10.6%, respectively, compared to that at the 2nd week. Conclusions This study provides a theoretical basis for the prevention and treatment of capsular contracture and development of rehabilitation aids.