Abstract: To realize the baseline motion and phrase control，many folding mirrors are used in optical trains of space stellar interferometers.The folding optical trains usually have very strong instrumental polarization.It causes polarization aberrations and thus decrease of imaging quality and fringe contrast of interferometers when the polarization properties for each arm aren′t identical to each other.So it is very necessary to analyze the polarization properties and optimize the polarization design of interferometers.A model of interferometric imaging of synthesis aperture system was built，considering the effect of instrumental polarization.The simulation method based on polarization ray tracing and the vectorial Fourier transform was developed to calculate the polarized interference fringe.An example Fizeau interferometer with three apertures was defined to implement the simulation.The result shows that the instrumental polarization of the given interferometer comes mainly from the delay line in each arm，which is composed of four diagonal mirrors.The profile and contrast of fringe are sensitive to the azimuthal angle of the incident plane of delay line.Setting the incident planes parallel to each other can decrease the polarization aberrations significantly，and making the imaging quality much closer to the ideal case.But in the visible band，this configuration can′t improve the fringe contrast to the required level for exoplanet detection.Generally，the instrumental polarization is an important factor of optical design of space interferometer.The polarization-optimized optical design is very necessary for these missions which pursue the astronomical high contrast imaging.

Key words: terrestrial planet detection, space interferometry, instrumental polarization, high contrast imaging