It is a new method for underground space development to upgrade the underground space of existing buildings. The existing supporting piles and newly added supporting piles constitute the composite supporting structure in the underground space downward storey increasing project. The bearing characteristics of the composite supporting structure in the whole storey increasing excavation process were studied by model test and finite element analysis. The research shows that the bearing capacity of existing supporting piles and newly added supporting piles is in a dynamic change with the excavation of additional layers. In the early stage of layer adding excavation, the existing supporting piles are the main load bearing members, while the newly added supporting piles play a role in embedding. In the middle stage of adding layer excavation, the load value of existing supporting piles and newly added supporting piles increases, and both of them are loaded members. In the later stage of the excavation, the bearing capacity of the newly added supporting piles is gradually greater than that of the existing supporting piles, and the existing supporting piles are gradually damaged, and the newly added supporting piles are the main load bearing members.