Abstract: The relative navigation of long-term following spacecraft in near-circular orbit was researched.For relative orbit modification at fixed time,the least-square estimation was used to derive the relative position and velocity of the fixed time.Based on the feature of model error,the algorithm was improved by weighted least-square.The new algorithm was simply and flexible.Numerical simulation result shows that the performance of the weighted least-square estimation is comparative with that of the Kalman filter,and the precision of the relative state estimation satisfies orbit modification for long-term following spacecraft.