Abstract: The retaining piles for foundation pits are usually arranged with equal pile lengths. However, in foundation pit projects in deep and soft soil areas, the retaining piles need to be embedded in good soil below the soft soil, so the pile lengths are longer. The economy is poor when the pile lengths are arranged with equal pile lengths. In order to examine the effect and performance of long and short pile combined support with partial retaining piles embedded in good soil in deep soft soil foundation pits, current design methods are used to calculate the structural internal force and deformation of retaining piles under the conditions of full length pile support and full short pile support. Therefore, it is estimated that the deformation of retaining piles under the condition of using long and short pile combined support should be between full length pile support and full short pile support. Through 3D numerical simulation, a comparative analysis is conducted on the combined support of long and short piles with different lengths to determine the appropriate length of short piles, and the analysis results are used to guide the actual project design and monitor the project. The research results show: (1) The length of the short pile supported by the combination of long and short piles can be taken as the second inflection point for calculating the bending moment of the retaining pile under the condition of full length pile support, achieving joint stress and coordinated deformation of the short pile and long pile. (2) The design and calculation method for long short pile combined support based on current design and calculation software presented in the paper is feasible. (3) The design method of long and short pile combination support can effectively save project costs. The monitoring data show that the application of long and short pile composite support in the project is successful, and the research results can provide references for deep soft soil foundation pit engineering.