The invention provides a method for decoding a bar code. The method comprises the following steps: a bar code reader acquiring an object image of the bar code; setting an object scan line on the object image; based on coding rules, analyzing a relative relation between at least two feature points on a gray scale distribution of the object scan line; when at least one reference feature parameter is in conformity with at least one first predetermined feature parameter, determining that a current distance between the bar code reader and the object bar code is a relatively long distance; when the current distance is determined to be the relatively long distance, dividing the gray scale distribution of the object scan line into at least one black bar area or white bar area with the predetermined gray scale value; based on the predetermined gray scale value, setting a gray scale section, finding at least one peak point or valley point positioned in the gray scale section from the gray scale distribution of the object scan line; when the peak point is positioned in the black bar area, inserting the white bar which corresponds with the peak point to the black bar area; when the valley point is positioned in the white bar area, inserting the black bar which corresponds with the valley point to the white bar area.