A security alert system adapted to be installed in an exhibition hall to exhibit a display content, comprising: a light emitting unit adapted to be installed in the exhibition hall for emitting a light beam consisting of an optical pattern onto a viewer, wherein said optical pattern varies depending on a distance between the viewer and the display content; an image capturing unit adapted to be installed in the exhibition hall corresponding to said light emitting unit, including an optical lens for capturing images of said optical pattern and the viewer and a photosensitive element coupled with said optical lens for sensing said images; a distance arithmetic unit adapted to be installed in the exhibition hall and coupled electrically with said image capturing unit for receiving said images from said image capturing unit and calculating a distance between the viewer and the display content based on variation of said optical pattern on said viewer of the received images; a multimedia player unit adapted to be installed in the exhibition hall and coupled electrically with said distance arithmetic unit such that when said distance between the viewer and the display content is smaller than a first threshold, said multimedia player unit displays multimedia information corresponding to said display content to be seen or heard by said viewer; and an alarm generating unit adapted to be installed in the exhibition hall and coupled electrically with said multimedia player unit such that when said distance between the viewer and the display content is smaller than a second threshold, said alarm generating unit generates an alarm signal to be seen or heard by said viewer, wherein said second threshold is smaller and/or shorter than said first threshold; wherein said light emitting unit is a flickering light source, in which the flickering frequency is the same as the frequency of said images captured by said image capturing unit, wherein said distance arithmetic unit further compares the image with and without the light emitting unit being lit for determining variation of said optical pattern. a light emitting unit adapted to be installed in the exhibition hall for emitting a light beam consisting of an optical pattern onto a viewer, wherein said optical pattern varies depending on a distance between the viewer and the display content; an image capturing unit adapted to be installed in the exhibition hall corresponding to said light emitting unit, including an optical lens for capturing images of said optical pattern and the viewer and a photosensitive element coupled with said optical lens for sensing said images; a distance arithmetic unit adapted to be installed in the exhibition hall and coupled electrically with said image capturing unit for receiving said images from said image capturing unit and calculating a distance between the viewer and the display content based on variation of said optical pattern on said viewer of the received images; a multimedia player unit adapted to be installed in the exhibition hall and coupled electrically with said distance arithmetic unit such that when said distance between the viewer and the display content is smaller than a first threshold, said multimedia player unit displays multimedia information corresponding to said display content to be seen or heard by said viewer; and an alarm generating unit adapted to be installed in the exhibition hall and coupled electrically with said multimedia player unit such that when said distance between the viewer and the display content is smaller than a second threshold, said alarm generating unit generates an alarm signal to be seen or heard by said viewer, wherein said second threshold is smaller and/or shorter than said first threshold; wherein said light emitting unit is a flickering light source, in which the flickering frequency is the same as the frequency of said images captured by said image capturing unit, wherein said distance arithmetic unit further compares the image with and without the light emitting unit being lit for determining variation of said optical pattern.