1. A method for warning a user about a distance between user's two eyes and a screen, comprising: acquiring an image containing the user's two eyes; obtaining a number of pixels between the user's two eyes in the image and comparing the number of pixels between the user's two eyes with a preset value; warning the user if the number of pixels between the user's two eyes is larger than the preset value, wherein the preset value is a preset distance between the user's two eyes, the method further comprising: converting the number of pixels between the user's two eyes into a distance between the user's two eyes in the image; comparing the distance between the user's two eyes in the image with the preset distance between the user's two eyes; and warning the user if the distance between the user's two eyes in the image is larger than the preset distance between the user's two eyes, wherein the distance between the user's two eyes in the image is d, the number of the pixels between the user's two eyes is n, a number in a length direction of a distance between the user's two eyes is n x , a number of pixels in a width direction between the user's two eye is n y , and d, n, n x and n y satisfy the relation: 
 d =√{square root over (( n x ×L x ) 2 +( n y ×L y ) 2 )} wherein, L x is a size of a single pixel in the length direction and L y is a size of a single pixel in the width direction.