It is the main task for the Chinese large solar telescope (CLST) with a 1.8 m aperture to measure the solar polarization with a high accuracy and sensitivity. However, the telescope system itself will introduce instrumental polarization. It also will change constantly with the rotating of the telescope and will reduce the accuracy. Therefore, a calibration unit is necessary to calibrate it. In this paper, we introduced the polarization calibration method and proposed a calibration unit structure.