An apparatus and a method for controlling backlight are provided. The apparatus for controlling backlight is adapted for driving a backlight module of a display panel, and the backlight module includes M luminance-controlling blocks, in which an ith luminance-controlling block corresponds to an ith luminance data. The apparatus for controlling backlight includes a calculation unit and a driving circuit. The calculation unit receives the ith luminance data to output an ith controlling data according to the proportion of the ith luminance data and a maximum luminance data, in which M and i are natural numbers, and i