The invention provides a dual-mode terahertz wave beam modulator based on a medium metasurface, a method and an application. The device comprises a plurality of 
metamaterial units arranged according to a certain rule, each 
metamaterial unit sequentially comprises a bottom VO2 layer, a 
dielectric isolation layer and a top VO2 layer from bottom to top, and centers of the bottom VO2 layer, the 
dielectric isolation layer and the top VO2 layer are superposed. The device is completely made of a 
dielectric material and does not contain a 
metal material, and the device can realize 
dynamic switching between a transmission mode and a reflection mode through 
thermal control or 
light control. In the reflection mode, the device efficiently reflects and splits terahertz 
waves, which is equivalent to a reflection type 
beam splitter. In the transmission mode, the device efficiently transmits the terahertz 
waves. Compared with other reflection type 
beam splitters, by using the device of the invention,when the device does not need to work as the 
beam splitter, an original transmission path of an 
incident wave is not changed, and the further transmission of the 
incident wave is not hindered. The device can be used for indoor terahertz communication so as to achieve the functions of communication range expansion, multi-terminal communication and the like.