In this paper, we propose a decoding approach for quasi-orthogonal space-time block code (QO-STBC) based on array processing. This approach works with single symbol and can deduce delay generated by the decoder as well as the decoding computational load. In this scheme, signals transmitted via different antennas are separated by null space, and then they are decoded linearly. Numerical results show that with the proposed decoding method the decoding complexity of quasi- orthogonal space-time block code can be greatly deduced. With 256-QAM modulation scheme, the computational complexity of the new decoder can be lower than that of the traditional decoder with 16-QAM.