When dynamic compaction method is used to improve the soft soil foundation,environmental vibration will appear due to radiating of compaction energy.Based on the wave-theory of semi-infinite elastic solid,attenuation of radial direction and vertical direction of Rayleigh with depth wave have been studied,and environmental influence is mainly decided by the vertical vibration of Rayleigh wave.Through in-situ test,attenuation law of acceleration and velocity of free ground,effects of vibration reduction by trench have been analyzed in this paper.