A wall plate assembly includes a wall plate with an insertion slot for the installation of a HDMI (High-Definition Multimedia Interface) connector of a HDMI cable and a power plug hole for the insertion of a power plug, a signal amplifier connected to the back side of the wall plate for receiving the installed HDMI connector and amplifying the signal, a support device, which comprises a first support frame vertically adjustably fastened to the wall plate below the insertion slot and a second support frame horizontally adjustably fastened to the first support frame for holding down the installed HDMI connector on the first support frame, and a power connector aimed at the power plug hole for receiving a power plug to provide the signal amplifier with the necessary working voltage.