An important scientific task of new vacuum solar telescope (NVST) is to obtain the middle and lower magnetic field of solar by accurate polarization measurement.In order to achieve high precision and high efficiency systematic polarization calibration,it is needed to build a polarization model for the telescope.This article analyzes the telescope instrumental polarization changes of coude optical path over days and seasons,and birefringence effects of its vacuum window.The result shows:the main crosstalk of Coude optical path is between linear polarization and circuit polarization,whose maximum number is 0.7.In the meantime,when the sun approaches midheaven around the summer solstice,the curve of crosstalk becomes shocked since dramatic derotation.And the total polarization property of vacuum window is equivalent to a retarder which changes along with the altitude of telescope.It becomes obvious when the telescope points are horizontal and the crosstalk increases to 1.2 × 10-2 .