1. An iterative decoding device comprising: a decoder for receiving a set of soft information, iteratively decoding the soft information and updating the set of soft information accordingly to generate a set of updated soft information; a dual-mode determination unit coupled to the decoder for generating a determination result, which indicates that a scaling operation is needed to be performed or not, according to a threshold value and the set of updated soft information, wherein the threshold value is set to a first threshold value or a second value based on a comparison result of comparing a scaling counter value, which indicates the number of times that the scaling operation is performed, and a predetermined value; and a dual-mode scaling unit coupled to the dual-mode determination unit and the decoder for generating the scaling counter value and performing the scaling operation that is to scale the set of updated soft information according to the determination result, so as to generate a set of scaled soft information acting as an input of the decoder for next iteration.