Since there would be a significant increase in missile guidance time constant and turning rate time constant when intercepting high-altitude and high-speed targets, compared with those at low altitude, small parasitic effect of seeker could probably cause large miss, resulting in failure of intercepting mission. To deal with this issue, the fifth-order linearized mathematic model was constructed on guidance and control system and both analytical ways and simulation ways were employed to investigate parasitic effect's relating factors and its influence on performance of proportional guidance. The results show that the parasitic effects of seeker constrain the guidance time constant, it means that a smaller guidance time constant must firstly loose the system parasitic effects and this could be reached by improving the missile's speed.