The spectral finite element method is used to analysis propagating wave types and their damping loss factor (DLF) in the beam with a non-constrained damping layer (NCDL). Every wave type’s exciting condition is studied by the analysis of characteristic wave shapes. The finite element method is used to compute the strain energy for every element of long beam with a NCDL, the DLF is computed by strain energy method and the relationship between structure DLF and load types is studied. In the end the boundary reflected waves’ effect on the DLF is studied. The study showed that: the vertical excitation mainly excites the highest order bending waves or lower order longitudinal waves whereas the longitudinal excitation mainly excites the highest order longitudinal waves; when the beam becomes longer and the displacement between the exciting point and the boundary becomes more far, the effect of boundary reflection on structure DLF become smaller; the DLF on different boundary condition are always the same except some special frequency; the structure DLF of a beam with a NCDL is decided mainly by the wave types and influenced a little by boundary reflection .