Taking the condition of underground space downward story-adding as background， the existing-added double-layer row pile support structure was taken as the research object. Robust stability of the existing-added double-layer row pile support structure during foundation pit excavation was analyzed by physical model test and finite element numerical calculation method. The conclusions are listed as follows. The destruction process of the existing support structure is gradual and there will be early warning before destabilization. The place that fails first in the existing support structure occurs randomly， which reflects that the uncertain factors have a great impact on the deformation of the support structure in the process of foundation pit construction. The soil between the two newly added rows of piles and the existing supporting piles acts like a gravity retaining wall. When the excavation reaches the critical depth， the integral overturning failure occurs. When the existing support structure is unstable， the sliding surface starts near the excavation surface， passes through the soil between the two rows of piles， and extends to the soil surface along the newly added support piles. The influence range of the sliding surface is the largest in the middle of the foundation pit slope and is the smallest at the pit corner. The local failure of the existing support row pile has little impact while the overall failure of the existing support row pile has a great impact on the robust stability of the support system.