A dynamic power adjusting device for a Viterbi decoder is disclosed. The device includes a processing unit for receiving a plurality of data to be decoded, detecting whether the data to be decoded have any bit errors, and estimating a number of the bit errors of the data. The device further includes a control unit for receiving the bit errors and the number of the bit errors of the data detected by the processing unit, so as to enable the Viterbi decoder to perform decoding, and disable the Viterbi decoder after the Viterbi decoder has performed the decoding the number of times equivalent to the number of the bit errors. By way of the dynamic adjustment technique, coupled with the bit-error detection and estimation, workload of the Viterbi decoder in operation is capable of being adjusted according to bit error rate (BER) such that less power is consumed in a decoding process.