Ambiguity fixing is the key to GNSS high-precision dynamic positioning. In order to analyze the performance of a GPS/BDS combined system on ambiguity search efficiency and the success rate of ambiguity fixing on a short baseline, this paper proposes an ambiguity search region correlation method based on the square roots of the eigenvalues of the variance/covariance matrix of ambiguity. This method compares and analyzes the ambiguity search region in a single system GPS,a single system BDS and combined GPS/BDS system, respectively. Experimental results show that the GPS and BDS combination influences the variance/covariance matrix of each single system, and reduces the search region of each single system. Furthermore, the statistical results show that the combined GPS/BDS system can improve the success rate of ambiguity fixing and the ambiguity search efficiency significantly within single- and dual- frequency observations.