Urban renewal refers to the transformation and upgrading of existing buildings and infrastructure in a city, and large-scale existing building renovation is often the key and difficult point in urban renewal. This article takes the Kunming Rongtong Center renovation project as a case to explore the digital route of large-scale existing building renovation design. The project has been vacant for nearly 25 years without any protective measures after the completion of its main structure. In order to reinforce and renovate it, the project team proposed a "five set model" system - environmental intact model, structural intact model, structural optimization model, full professional low-carbon optimization model, and digital twin model. Through the digital technology path of large-scale existing building renovation design based on five sets of model systems, promote the feasibility and reliability of the renovation, greening, and digital construction of large-scale existing buildings.