Abstract: The seeker disturbance rejection rate not only severely degrades seeker tracking accuracy, but also has greatly issues for the missile guidance performance. The generation mechanism of disturbance rejection rate was analyzed, and the models of disturbance rejection rate caused by spring torque and damping torque were established. Based on the Routh criterion, the stability zone of parasitical loop was obtained. The effect of the parasitical loop on effective guidance time constant and effective navigation ratio was studied. The result shows that the limits for important guidance parameters should be determined to insure the stability of parasitical loop; both the effective guidance time constant and effective navigation ratio are changed on the action of parasitical loop. The conclusion is helpful for engineers at the preliminary stage of guidance and control system design.