The shallow layer of an office building site was reclaimed by backfilling, the thickness of filling layer was about 10m with particle size of 20~80cm. To meet the requirements of basement structure and pile foundation construction, generally the excavating depth was not less than 8.7m and the local depth reached 14.4m. It is particularly important to select proper foundation pit support plan for the very difficult support in this project. Step-slope excavating and water-sealing curtain scheme was chosen, the safety of the foundation pit and the surrounding buildings were ensured in the design calculation. The results show that this selected scheme is reasonable, safe and reliable.