Abstract: The problems on disturbance rejection rate of strapdown imaging seeker and the stability of disturbance rejection rate parasitical loop were analyzed in this paper. Firstly, two calculation methods of line of sight (LOS) rate were given based on the information from seeker and inertial measurement unit (IMU). The calculation model got simplified in order to analyze the influence to disturbance rejection rate from different disturbance items. Then the magnitude and phase of disturbance rejection rate caused by the aerodynamic model, time delays of signals and graduate scales were analyzed, especially in the low frequency bandwidth. Finally, the disturbance rejection rate parasitical loop model based on strapdown seeker was derived. Through discussing the stability zones of parasitical loop, the permitted maximum values for different disturbances were given. The results present that disturbance sources mention above will cause the terrible disturbance rejection rate problem and make the parasitical loop become unstable, so it is necessary to rectify them when calculating LOS rate.