Abstract: This paper presents a mathematical model of determining states of three satellites circular formation constellation, which combines the Carrier Phase Differential GPS (CDGPS) with inter-satellite relative measurement. And then, with the centimeter length constraint to the kilometre baseline inter-satellite which supplied by high accurate inter-satellite relative measurement, the search space of ambiguity is compressed impressively. On this base, the integer ambiguity of single CDGPS inter-satellite was initialized quickly using Bayes Least Square without any satellite movement intentionally. Lastly, the availability was proved by mathematic simulation. Learning from the simulation result, the accuracy of relative positions between satellites reaches 10^-2 m and the accuracy of attitude of satellite reaches 10^-3 rad.