An interlocking bone plate system includes an outer bone plate for being arranged outside a broken bone, an inner bone plate for being installed inside the medullary cavity of the broken bone, and screws for being inserted through and engaged with the outer bone plate and the broken bone and then engaged with the inner bone plate so as to interlock the out and inner bone plates together. The inner bone plate provides an added support in addition to the support provided by the outer bone plate, enhancing the structural strength of the whole bone fixation structure and lowering the risk of failed surgery.