The US GPS and the planed Quasi Zenith Satellite System (QZSS) will enhance the capability of quickly resolving the integer cycle carrier phase ambiguities in precise differential positioning. The paper describes numerical analysis of the positioning performance of combined GPS-QZSS system. The current ambiguity resolution method is employed to resolve all cycle ambiguities of combined GPS-QZSS system. The performance of ambiguity resolution on a short baseline of 1 km in Tokyo is analyzed for different scenarios of the present GPS and combined future GPS-QZSS system. It is also analyzed in the Asian cities of Seoul, Beijing and Shanghai. The ambiguity fix percentage is adopted here for evaluating the performance of ambiguity resolution. It indicates that increasing the number of satellites has a benefit on the capability of getting quick ambiguity resolution. It also indicates that the ambiguity fix percentage with both GPS and QZSS is much higher than that with only GPS on the short-baseline operation.