An exemplary automatic focusing system adopted in a camera includes a brightness detecting unit, a recording unit, a correcting unit, and a selecting unit. The brightness detecting unit is configured for detecting a general brightness value of an environment of a subject prior to a focusing operation of the camera and outputting a correction factor associated with the general brightness value. The recording unit is configured for storing a first set of brightness values and the first set of contrast values. The correcting unit stores a preset brightness value and a preset contrast value. The correcting unit is configured for calculating a number of second contrast values according to the correction factor, the first set of brightness values, a preset brightness value, and a preset contrast value. The selecting unit is configured for choosing a maximum contrast value from the number of second contrast values of the subject.