In order to reflect the output characteristics of real seeker under disturbance rejection rate, a concept and test method of seeker transfer function based on disturbance rejection rate parasitic loop were proposed. Through the establishment of seeker disturbance rejection rate model, the transfer function of disturbance rejection rate with effect of different disturbances was derived. The stability region of disturbance rejection rate parasitic loop was analyzed, and the regulation of parasitic loop instability frequency was obtained. The transfer function model of seeker with parasitic loop was established, and the frequency characteristics and time domain characteristics of the real seeker model were simulated and analyzed. Finally, the hardware-in-loop simulation test method of seeker transfer function based on parasitic loop was proposed. Through the hardware in the loop simulation test, the real seeker model is obtained, so as to improve the accuracy of seeker modeling.