Multiple-input multiple-output (MIMO) system has very high spectrum efficiency. However, detection is a major challenge for the utilization of MIMO system. But even the fixed sphere decoding (FSD), which is known for its simplicity in calculation, requests too much computation in high order modulation and large number antenna system, especially for mobile battery-operated devices. In this paper, a reduced FSD algorithm is proposed to simplify the calculation complexity of the FSD while maintaining the performance at the same time. Simulation results show the effect of the proposed algorithm. Especially the results in a 4×4 64QAM system show that up to 81.2% calculation can be saved while the performance drop is less than 0.1dB when SNR=30.