Abstract: The dynamic interaction problems of three-dimensional lineqr elastic structures with arbitrary shaped section embedded in a homogeneous,isotropic and linear elastic half space under dynamic disturbances are numerically solved.The numerical method employed is a combination of the time domain semi-analytical boundary element method(SBEM) used for the semi-infinite soil medium and the semi-analytical finite element method(SFEM) used for the three-dimensional structure.The two methods are combined through equilibrium and compatibility conditions at the soil-structure interface.Displacements,velocities,accelerations and interaction forces at the interface between underground structure and soil medium produced by the diffraction of wave by an underground structure for every time step are obtained.In dynamic soil-structure interaction problems,it is advantageous to combine the SBEM and the SFEM in an effort to produce an optimum numerical hybrid scheme which is characterized by the main advantages of the two methods.The effects of the thickness,the ratio of length and diameter of underground structure and the soil medium on dynamic responses are discussed.