A display device and a controlling method are disclosed to achieve peep-proof effect while increasing utilization rate of light. The display device includes a display panel including a first substrate, a second substrate and a plurality of display units, wherein a first light source is disposed on one side of the first substrate; light emitted by the first light source is incident onto the first substrate and propagated in the first substrate in a manner of total reflection; and a light adjusting structure is disposed on a surface of the first substrate close to the second substrate, and is configured to reduce a divergence angle of light emitted by each of the display units of the display panel.