Due to the strapdown image seeker cannot measure the inertial line-of-sight (LOS) rate directly, an arithmetic for estimating the LOS rate based on extended Kalman filter(EKF) was presented. The LOS rate decoupling expression under ideal condition was deduced. The model for estimating available guidance information based on EKF equations was established. Then according to the features of operational use of the man-portable guided munitions, the hit probability was analyzed by Monte Carlo method, the Circular Error Probable(CEP) met the operational requirements. The engineering feasibility of guidance system is verified by hardware-in-the-loop simulation. The results show that the arithmetic is an effective solution for estimating available guidance information for strapdown image guided man portable munitions. The technical approach of strapdown image seeker with estimation arithmetic of guidance information is suitable for engineering application.