The interaction between hammer and formation is a main concern in the dynamic compaction studies. According to the week cemented and porosity characteristics of the unsaturated soil under impact loading, the elasticity behavior of the unsaturated soil skeleton is not important. In this paper, the momentum equation between the hammer and the soil layers is established, and the dynamic compaction is expressed as a process of the layers reaching the ultimate porosity sequentially. The recursive equation based on the above viewpoint is also set up and extended to the whole process of the dynamic compaction. Based on the recursive equation, a computation program is developed, and furthermore, the foundation settlement, the hammer speed, the impact time and others are calculated for the example of the dynamic compaction project in Beijing Garden EXPO. It is shown that the total settlement increases with the impact number, the single settlement decreases with the number and appears to be stable after several impacts. The calculated settlement is in good agreement with the field experimental data. The tamping speed and the settlement with the tamping time are discussed in the end. The momentum recursive method provides a new way for the calculation of the dynamic compaction in the unsaturated under-consolidated soil and could be used in engineering designs.