The degradation characteristics of high-purity (HP) magnesium (Mg) orthopedic implants under static and cyclic compressive loads (SCL and CCL) remain inadequately understood. This study developed an in vivo loading device capable of applying single SCL and CCL while shielding against unpredictable host movements. In vitro degradation experiments of HP Mg implants were conducted to verify the experimental protocol, and in vivo experiments in rabbit tibiae to observe the degradation characteristics of the implants. Micro-computed tomography and scanning electron microscope were used for three-dimensional reconstruction and surface morphology analysis, respectively. Compared to in vitro specimens, in vivo specimens exhibited significantly higher corrosion rates and more extensive cracking. Cracks in the in vivo specimens gradually penetrated deeper from the loading surface, eventually leading to a rapid structural deterioration; whereas in vitro specimens exhibited more surface-localized cracking and a relatively uniform corrosion pattern. Compared to SCL, CCL accelerated both corrosion and cracking to some extent. These findings provide new insights into the in vivo degradation behavior of Mg-based implants under compressive loading conditions.