Through the study of grading analysis and compacting test, this article analyzes the physical and mechanical property and the compacting capability of silt. As a result, it is one of the important reasons for the highway damage that the criterion of compaction in existence is deficient. In order to solve this problem, volume ratio of air is used as an assistant criterion for the compaction of silt. And, it is suggested that the degree of compaction for the road embankment should be improved.