Under the condition of not changing the appearance and adjacent landscape, a new built-in underground structure scheme is adopted in a historic conservation building in Beijing to realize the reconstruction of the long-span structure of the existing basement, increase the height of the basement and external expansion. In order to carry out the reconstruction of large-span structure, the permanent underpinning structure, which combines underpinning structure with newly-built underground structure is adopted to realize the integral underpinning of the internal load-bearing structure of the existing building, and the secondary deformation of the existing structure caused by the temporary underpinning structure and its removal is avoided. In order to increase the height of the basement, the load of the permanent underpinning structure and the load of the existing structure are transferred to the underpinning piles of the lateral underpinning structure of the foundation on both sides. The excavation of soil, the construction of lateral underpinning structure of foundation, and the construction of new underground structure under permanent underpinning are carried out sequentially. While retaining the stress and deformation of the existing foundation and controlling the settlement of the existing building caused by soil excavation, the lateral underpinning structure of the foundation plays an intermediate transfer function for the load of the permanent underpinning structure. This paper introduces the project implementation from the aspects of new built-in underground architectural and structure scheme, load underpinning and transfer system, underpinning structure design, structural stability, settlement control, settlement monitoring and so on. The new built-in underground structure and load underpinning technology adopted in this project is of universal application value and can be widely used in the underground space expansion project of existing buildings.