1. A system for modulating backlight, comprising: a light source module providing backlight for a display panel and forming a plurality of illumination areas corresponding to a display area of the display panel, wherein the illumination areas are adjacent to each other and substantially cover the display area; an image mapping unit performing a transformation on an image displayed on the display panel to convert a RGB color model of the image to another color model independent of an image processing device to acquire an original brightness factor for each pixel of the image; a histogram analysis unit summing up the amount of total pixels of each illumination area and calculating the amount of accumulated pixels in each illumination area reaching a preset ratio to acquire a reference brightness for each illumination area; a backlight dimming unit calculating a backlight dimming ratio and a mapping calibration curve of each illumination area according to the reference brightness of each illumination area, wherein the backlight dimming ratio BD(n) of the n th illumination area satisfies the following criterion: 
 BD ( n )= Var+[ABL _( n )/( ABL MAX /(1 −Var ))], where Var is a variable larger than 0 and smaller than 1, ABL_(n) is the reference brightness of the n th illumination area and ABL MAX is the maximum reference brightness; when the light source module comprises N a light sources to divide the display area into (N a +1) illumination areas, if each illumination area is distributed with N p scan lines, the slope of the mapping calibration curve of all the N p scan lines in the first illumination area is the inverse of the backlight dimming ratio of the first light source, the slope of the mapping calibration curve of all the N p scan lines in the (N a +1) th illumination area is the inverse of the backlight dimming ratio of the N a th light source and the slope S(m,n) of the mapping calibration curve of the m th scan lines in the n th illumination area of the 2 nd to (N a ) th illumination areas satisfies the following criterion: S ⁡ ( m , n ) = 1 BD ( n - 1 ) + ( BD ( n ) - BD ( n - 1 ) ) × m Np , where BD (n) represents the backlight dimming ratio of the n th illumination area, BD (n-1) represents the backlight dimming ratio of the (n−1) th illumination area, 2≦n≦N a , 1≦m≦N p , and m and n are positive integers; and an image reconstruction unit resetting the original brightness factor of each pixel in an illumination area into an output brightness factor according to the mapping calibration curve of the illumination area and outputting an image for the corresponding illumination area according to the output brightness factor and input image data.