Vector Viterbi Algorithm (VVA) is an optimal decoding algorithm in multiplechannel environments. It can be applied in coded Multi-Input-Multi-Output (MIMO) systems,but its complexity makes its application difficult. This paper proposes adaptive Threshold VVA (T-VVA) for coded MIMO system. By adaptively choosing a threshold according to Signal to Noise Ratio (SNR) of channel, T-VVA only searches a subset of branchesfor path extension. The easy method to decide the threshold is also derived based on Cheroff bound. Simulation results show that the near-optimal decoding results can be obtained with largely reduced complexity.