The disturbance rejection rate problem caused by strap-down seeker was studied, the parasitical loop model induced by strap-down seeker time delay was established, the principium of how time delay causes disturbance rejection was analyzed, and the function of disturbance rejection rate induced by strap-down seeker time delay was deduced. Moreover, stability of the parasitical loop caused by time delay was investigated and of the parasitical loop with different guidance parameters was illustrated. The simulation result shows that, if the guidance system lag increases relatively to the missile aerodynamic time constant, the system can overcome larger seeker time delay; if the missile aerodynamic time constant is reduced, the guidance can tolerate larger seeker time delay. Disturbance rejection rate problem discussed in the paper is significant in the practical utilization.