Abstract: A large field test was conducted in the project of Chang-Ji Expressway.Forty-three soil pressure cells were embedded in the test section of road.Dynamic strain gauge was used to record the stress dispersion and stress decay caused by dynamic compaction.It was tried to explain the spread of shock waves in the soil mass with the stress dispersion and stress decay curve.It was shown that the reinforce effect on the red sandstone fill in subgrades and dynamic stress of dynamic compaction were obvious.The decay speed of dynamic stress in the horizontal was faster than that in the vertical,and the range of influence of dynamic stress was much extensive than that in the horizontal.The effective reinforced depth was deeper than the influence depth.With the compaction numbers increased,the dynamic stress in the reinforced range obviously increased and it tended to be stable after three to five hits.The effective range of P-wave,S-wave,R-wave could be analyzed from the contours of dynamic stress.