A metal PPWG in the THz regime is intensively studied due to its superior confining effect for the propagating THz waves. Here an observation of an EIT-like phenomenon in THz PPWG cavities systems and analyze the relation between the off-position of the cavities and the transmission properties. In addition, we also discovered the understanding of the surface cavity mode and high order cavity mode in single long cavity, as well as the mutual transition between them, which show applications in terahertz filters with adjustable channels.