Instrumental polarization is a vital factor for the accurate measurements of the solar magnetic field. It is necessary to optimize the optical design of the large solar telescope to obtain high accuracy solar magnetic field information. In this paper, an optimal design scheme based on four-mirror polarization compensation optics for the 8-meter giant solar telescope is presented. The polarization effect of pupil and field of view are analyzed by the polarization ray tracing programming, the telescope motion and wavelength properties of the field-effect are investigated detailedly. The result shows that, in the near infrared waveband which includes the magnetic sensitive spectral lines of He I 1.083μm and Fe I 1.565μm, the polarization-free field size is 0.91′, and the polarization-free field size in visible is 0.5′, in which the instrumental polarization of telescope is smaller than 2x10^-4.