A display device comprises a backlight module, a display panel and a first glue layer. The backlight module comprises a light guiding plate, a flexible reflecting sheet and a light source. The light guiding plate is provided with a light incoming face, a light outgoing face, a back face and a plurality of side faces. The flexible reflecting sheet is provided with a first area segment and a plurality of second area segments. The back face of the light guiding plate leans against the first area segment; the second area segments are connected to the first area segment and are bent relative to the first area segment; and at least part of the second area segments cover the side faces of the light guiding plate correspondingly. The light source is arranged on the flexible reflecting sheet and is aligned with the light incoming face of the light guiding plate. The light outgoing face faces the display panel. The display panel and the backlight module adhere to each other through the first glue layer. The first glue layer is located between the display panel and the second area segments of the flexible reflecting sheet. The invention also discloses an assembling method of the display device. Thus, the display device can achieve the frame narrowing function.