With the development of the technology of tightly integrated INS/GNSS, which is widely applied to the formation relative navigation, Combing the relative navigation technology based on GNSS difference with the technology of INS/GNSS integrated navigation, the relative navigation algorithm of tightly integrated INS/GNSS is researched in this paper, regarding the double-difference of pseudo-range/pseudo-range rate as observables. The most of transmission pathway errors and common time errors can be eliminated by two times differential operators, and a part of measurement noise by the smooth properties of INS. Simulation results show that the algorithm can obtain the more higher accurate and steady relative navigation information.