In the sphere decoding (SD) algorithm, the strategy of initial radius selection and assignment has significant impact on the performance and complexity of the algorithm. In order to reduce the prohibitive complexity of conventional SD algorithm at low signal-to-noise ratios (SNRs), a novel strategy of initial radius selection and (quasi-) linear assignment is proposed based on V-BLAST signal model. The method of analyzing the performance and complexity of SD algorithm applying the proposed scheme is suitable for any nonuniform radius assignment strategy. Analysis and simulation show that, compared with the conventional SD algorithm, the complexity is reduced significantly by the proposed scheme at low SNRs, and the bit-error-rate performance is close to that of maximum likelihood detection in a relatively large SNR range.