In order to determination fixed optical path difference of wind imaging interferometer,the principle of wind field measurement is described. The factors which influence on fixed optical path difference are analyzed. Three requirements including visibility,phase and near spectral lines in phase for WINDII(Wind Imaging Interferometer)are analyzed respectively and obtain quantificational relations between them and fixed optical path difference. Three conditions are considered together and conclude the selective principle of fixed optical path difference. At last the theory is applied to other wind imaging interferometers and proves their fixed path differences.