The outdoor stone furnishings of the Forbidden City exist in various types and large quantities, but their preservation status is unsatisfactory, with some furnishings exhibiting serious diseases. Conducting evaluations of the preservation status of these cultural relics constitutes a prerequisite for effective conservation. In this study, we analyzed the disease conditions, physical properties, and related data of the stone furnishings of the Forbidden City to evaluate the overall preservation status of marble artifacts. On this basis, current evaluation methods for the preservation status of stone cultural relics were assessed, and a new evaluation method—the “average analysis method”—was proposed. Compared with the commonly used ones, this new method demonstrated higher accuracy in evaluating the preservation status of stone cultural relics.