Abstract Whether the initial search space is set reasonably or not directly affects the reliability and search efficiency of ambiguity resolution. In view of the constellation characteristics of BDS system, it is necessary to analyze the correlation between ellipsoid volume and the number of candidate points, and the effect of data redundancy for BDS2 and BDS3 under different satellite numbers and frequencies. For the drawback that objective function method is prone to produce too many candidate vectors when determining the ambiguity search space, we propose an optimized search space determination method, and verify the effectiveness of improved method through experiments. The results show that the improved search space determination method significantly reduces the number of actual ambiguity candidate vectors, with average improvement up to 35.02%, and ensures that the number of candidate vectors contained in 94% space satisfies the requirements. The improved search space method improves search efficiency by nearly 33% on average after applying it to different search methods, which is conducive to separating the redundant candidate vectors in initial search space, thus improving the ambiguity search efficiency.