It is of distinct advantage that the members of an air fleet are equipped with just inertial navigation system (INS) in collaborative formation flying, such as concealment, anti-jamming,etc. However, with the increase of endurance, INS error will be cumulated continuously, which may lead the navigation system to fail. Therefore, this article proposes a joint correcting method of multi-platform INS errors based on relative navigation. First, it builds a relative motion model and a nonlinear observation model, constructed a pseudo linear observation model with measurement reconstruct technique, and then it deduces the pseudo linear observation covariance matrix. Second, the article proposes a joint correcting method of multi-platform INS errors based on relative navigation, and demonstrates the satisfactory effect of this joint INS error correction. Furthermore, it provides a specific strategy of joint correction. Finally, the simulation result shows that compared with the extended Kalman filter (EKF), the proposed method can increase the convergence rate of relative navigation by a large amount while the accuracy remains unchanged on the whole. The joint correcting method of multi-platform INS errors based on relative navigation effectively slows down the divergence speed of INS errors of a formation system.