Mandibular mobility plays a significant role in human daily lives, enabling food intake, respiration, speaking, and other oral activities. However, as for the patients suffering from mandibular mobility disorders, their mandible functions are deteriorated, which severely affects their quality of life. In this paper, we present a new solution to recover mandibular mobility: A soft oral rehabilitation robot (SORR), which is actuated by a novel type of soft pneumatic actuator (SPA). After identifying the biometrics of the human mandible, we illustrate the application-oriented design of the robot and the SPA. A static model of the SPA was established to predict its behavior and eligibility for the application. In the experimental characterization, we measured the elongation and force output of the SPA, and the detailed results are presented. The comparison and analysis of the results provide physical insight into the mechanisms of the SPA. As the first application of soft robot inside human body, this work enlightens profound application potentials of the silicone-based SPAs.