Taking the deep soft soil subgrade in the railway special line of a port area in Beihai as the engineering background, this paper considers comprehensive factors such as site conditions, construction period, and project cost and puts forward the technical scheme of soft foundation treatment by dynamic compaction cushion method. Through the field full-scale test of soft foundation reinforcement, the paper verifies the effect of the dynamic compaction cushion method on soft foundation treatment by static cone penetration test, standard penetration test, and load test. The results show that the proposed technical scheme of the dynamic compaction cushion method is feasible, and it can deal with the soft soil subgrade with a thickness of 6 m. The dynamic compaction cushion method combining point compaction and full compaction can effectively improve the efficiency of soft foundation treatment. The research results can provide technical references for similar soft foundation reinforcement projects.