Based on the radar phased array beam control theory and the stable tracking technique of the platform seeker, the control system model of the phased array seeker with platform (PASP) is established. Platform rate feedforward is used to decouple the line of sight (LOS) rate from projectile disturbance. According to different feedforward signals, the corresponding disturbance rejection effect (DRE) and parasitic circuit are studied. The study shows that the DRE of PASP is much smaller compared with strapdown seekers, and the stability of the parasitic loop seems better when the output of gyroscope is selected as the feedforward signal. Considering the typical random noises, the parasitical loop effects on the guidance system are analysed using the adjoint method. The results show that when using the structure of the platform and choosing the gyroscope output as the feedforward signal, and the phased array seeker can effectively reduce the influence of DRE on terminal guidance and achieves a better performance.