This paper studied the influence of the total error of phased array strap-down seeker, the total error includes beam pointing error, radome error, and scale-factor uncertainty. The mathematic model considerating the total error slope is established firstly. Based on that, the transfer function model of the parasitic loop with the total error slope is established. In order to mitigate the influence of the total error slope for the seeker, a method which based on the loop shaping method has been put forward. The study via adjoint method showed that using the proposed method in this paper can increase the stability of the guidance loop and mitigate the influence of the total error slope effectively.