This paper studies the derivation and implementation of an algorithm for estimating the angular rate of Line of Sight (LOS) angle between guided munitions and targets.The measurement information of a low-cost strapdown seeker includes two parts, namely, attitude of the missile and LOS angle, which are formed in relative motion between the missile and the target.Firstly, a decoupling algorithm is designed to remove the attitude information of the missile.Then, Extended Kalman Filter (EKF) and α-β filter are used to estimate LOS angle and angular rate respectively.The final simulation results show that EKF can obtain a more accurate estimated value than α-β filter.