Roll-pitch strap-down seeker has good application prospect.However,the problem of extracting LOS (Line-of-Sight) rate steadily during zenith pass tracking has not been properly solved.We proposed an approach for constructing a virtual “pitch-yaw” seeker by using the seekers angular position error,gimbal angles and gimbal angle rates.A Kalman filter was also designed whose input is the measurement of the virtual seeker to estimate movement information of the target.And the LOS rate could be extracted indirectly.Simulation results show that the proposed method can restrain the dithering of the LOS rate efficiently during zenith pass tracking and improve the precision of LOS estimation.