Based on the project of underwater complex asymmetric excavation of deep foundation pit in Nanchang Aixihu overlapped tunnels, the deformation difference of supporting piles on both sides of a typical section is analyzed, and it is pointed out that adjusting the design length of supporting piles in a certain range has little effect on deformation, the pile length can be optimized to improve economic benefits. Then, based on the simplified model,  the mechanical response law of foundation pit supporting structure under different influence factors is analyzed, it is pointed out that asymmetric excavation foundation pit supporting structure should use asymmetric supporting length, and the concept of optimal insertion ratio of supporting pile is put forward combined with the engineering practice. Last, the effects of the second bracing position, different bracing stiffness, different supporting pile stiffness, and different soil on the optimal insertion ratio are analyzed, and the optimal design principle of foundation pit supporting pile is put forward. The result shows that the foundation supporting pile at the asymmetric loaded side can be optimized by adjusting the designed length at the side with smaller load, and the deformation can be properly controlled by adjusting the designed stiffness of the supporting structure. The conclusion has been applied in practical engineering, the design length of supporting pile has been optimized, and certain economic benefits have been obtained.