A cable straightening device includes a plurality of cable straightening units connected in series. Each of the cable straightening units includes a base, a pair of rows of rollers mounted on the base and cooperating with each other to define a pressing path extending in a row direction, and a mounting frame disposed on at least one end of a pair of opposite ends of the base in the row direction. A cable is pressed by the rows of rollers while passing through the pressing path. Any two adjacent cable straightening units are assembled together at the mounting frame of each so that center lines of the pressing paths of two adjacent cable straightening units are aligned with each other.