In a project of backfill collapsible loess foundation in the Northeast, some regional soil collapsibility was not completely eliminated. Analysis is made on the influencing factors on effective reinforcement depth by dynamic compaction method in backfill collapsible loess foundation, the first influencing factors are determined to be the consolidation energy and one-shot points, and the soil moisture content, the thickness and distribution of different soils, compaction rate and the dynamic compaction design factors should also be considered.