Abstract: For issues of the disturbance rejection rate parasitical loop (DRRPL) of strapdown phased array radar seeker, the reasons for the problem were discussed. The normalized model of DRRPL was established, and the stability and the frequency response of DRRPL were analyzed. The effect of DRRPL on PN guidance system was studied with the method of adjoints. The results demonstrate that the imperfect decoupling and the error of angle measure are main reasons for the above problem of DRRPL. Compared with the feedback, the parasitical loop with the feedforward is more sensitive to disturbance rejection rate, and the open-loop crossover frequency gets lower when DRRPL is marginally stable. Important parameters and miss distance (MD) of PNG will be affected seriously by DRRPL.