Abstract: An adaptive Viterbi decoder is designed in this paper, based on the demand of adaptive and low power consumption in modern communication systems. The Viterbi decoder can change trace depth automatically by the design of a reconfigurable survivor path memory unit. In this paper we also simplify the branch metric arithmetic, so as to decrease the complexity of the branch metric unit and the add-compare-select unit. According to the simulation result, this design has a small area and has satisfied the demand of adaption, the power consumption can be decreased by 14％. The Viterbi decoder supposed in this paper can be used in communication systems which have kinds of modulation modes.