To study the effect of phantom-bit technology for phased array radar seeker （PARS） on missile guidance system, the cause of boresight error slope （BSES） is discussed according to the principle of antenna element shift phase, the transfer function of line-of-sight rate containing BSES is derived under the disturbance of missile body, and the change rules of amplitude and phase angle are analyzed. A model of BSES parasitic loop is established based on the pointing angle of beam space, and the effect of phantom bit number （PBN） on the stability boundary of parasitic loop is studied with Routh criteria. Under the condition of the typical radar noise input, the dimensionless and adjoint methods are used to research the influence of PBN on miss distance. The result shows that phantom-bit technology of PARS leads to BSES ; the larger the PBN is, the bigger the BSES is, the easier the parasitic loop loses its satbility, and the larger the miss distance is. The receiver noise has a greater effect on miss distance compared to target glint noise under the same condition. Therefore an error calibration method, which is used to calibrate the error among different beam angle directions according to different PBNs, needs to be studied to reduce BSES and improve the accuracy of missile terminal guidance.