Abstract: To compensate for the current existing single-epoch algorithm to resolve ambiguity, which is only suitable for short baselines, we propose an algorithm with dual-frequency GPS observations to achieve medium and long baseline single-epoch ambiguity fixed according to the error propagation rate. Then, the ambiguity search space is determined according to the accuracy. Firstly, the linear constraint condition between the double-difference ambiguities is obtained by the ionospheric constraint method. Secondly, the approximate coordinates of the monitoring station in the deformation monitoring are used to obtain the approximate ambiguity. Finally, the search space is obtained according to the accuracy of the observational data, and then we search for and fix the ambiguity. The experimental results show that the method can fix the ambiguity of single epoch under medium and long baseline, and it has certain reference value for real-time deformation monitoring.