Space-based Automatic Identification System (AIS) uses Gaussian minimum shift keying modulation (GMSK) as the modulation scheme. Thus, Viterbi Algorithm (VA) is commonly implemented considering the tradeoff between performance and complexity. For the further improvement of the performance of Viterbi decoding, we propose a Soft-Output Viterbi Algorithm for AIS receiver (SOVA-AIS) in this paper. The proposed SOVA-AIS introduces soft values to quantify the error probability of demodulated bits and use them to update the survivor path in VA to get more precise estimation of the signal phase. Simulation results show that, compared with VA, SOVA-AIS can provide 0.5-1.5dB gain in the case with only one AIS signal and 1-3dB gain in the case with two collided AIS signals.