We propose an algorithm that reduces the complexity of the K-best sphere decoding (K-best SD) algorithm, which is a powerful parallel detection algorithm for multiple-input multiple-output systems (MIMO). By analyzing the probability of different nodes to be the final solution, the algorithm prunes some nodes during the tree search to reduce the complexity. Simulation results prove that compared with the K-best SD algorithm the proposed algorithm performance drops very little. Compared with the famous fixed-complexity sphere decoding (FSD) with the same complexity, the proposed algorithm has better performance.