An iterative decoding device applied for a SISO (soft input soft output) system is disclosed, which comprises an operational control unit, a first decoder, and a second decoder. The operational control unit is operative to receive an encoded signal and divide the encoded signal into at least one frame. The first decoder is operative to receive each of the at least one frame and derive a renewed intrinsic information by a first iteration operation. The second decoder is operative to derive soft-information by a second iteration operation based on the renewed intrinsic information, and then transmit the soft-information back to the first decoder for the iteration operation of the next renewed intrinsic information. The operational control unit makes the at least one frame to be calculated respectively by the first decoder and the second decoder, thereby improving the efficiency and error ratio of a receiver.