Abstract: In this paper, a method for fast ambiguity resolution between reference stations with GNSS zero baseline constraints is proposed. The method is based on the design of "one antenna+two receivers". First of all, using the observations of four receivers on two reference stations, the double difference observation of the ionosphere free combination between four reference stations is constructed. Secondly, combined with the ionosphere ambiguity and the ambiguity of the whole lane, the real solution of N1 ambiguity is obtained by Kalman filter. Then, the linear transformation in the ambiguity domain, single epoch fixed zero baseline ambiguity; the relationship between the use of constraints of zero baseline ambiguity, calculated in the four groups a set of reference station ambiguity. Finally, all the original double difference ambiguities are solved by using the fixed ambiguity. Experiments show that compared with the traditional reference station, the ambiguity fixing method, make full use of the constraint of zero baseline ambiguity, accelerate the ambiguity speed, greatly improve the ambiguity speed and success rate.