Based on the composite foundation treatment project of the collapsible loess area in G6 Beijing-Tibet Expressway Lanzhou-Haishiwan Section, using FLAC3D numerical analysis method to calculate the settlement of composite foundation under the action of vehicle load and composite foundation weighting, the numerical analysis results are verified by combining the settlement observation data of the actual project. The results show that the settlement of final monitoring of the composite foundation is approximately 15mm, which is basically consistent with the numerical analysis results. As the pile depth inserts into the soil gradually increases, the final settlement of the pile is also significantly reduced when the stress balance is reached. The whole settlement process can be divided into two time periods. In the first 1/3 period, the settlement period is the largest, and in the second 2/3 period, the settlement amplitude drops sharply.