Digital restoration of ancient architecture is a key issue in the preservation and utilization of historical buildings. Traditional research has mainly focused on above-ground structures. In Chinese ancient architecture, scholars have emphasized timber-frame materials and craftsmanship. However, studies on the masonry and stonework of foundations remain limited. This gap represents the main focus of this study. Located in the core section of Beijing’s Central Axis, the Diji Altar is an important historical landmark. It is also a significant surviving example of ancient Chinese imperial ritual architecture. Existing studies have examined certain aspects of the Diji Altar. These include its ritual status and craftsmanship, as recorded in historical texts. Nevertheless, notable research gaps persist. The altar structure has sustained damage. Moreover, historical records lack sufficient documentation on its structural system, the form of its foundation platform, and corresponding stylistic evidence. As a result, systematic research on restoration techniques remains scarce. Such studies must rely on inferences based on architectural codes. Addressing this critical gap holds great significance. It contributes to understanding the technological achievements and ritual concepts of official architecture during the Ming and Qing dynasties. It also provides guidance for the restoration and preservation of ancient architecture.