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Polarization calibration unit (PCU) has become an indispensable element for solar telescopes to remove the instrumental polarization;the polarimetric accuracy of calibration depends strongly on the properties of PCU.In the paper,we analyze the measurement errors induced by PCU based on polarized light theory and find that the imperfections of the waveplate generate the main calibration errors.An optimized calibration method is proposed to avoid the effects from waveplate imperfections,and a numerical simulation is given to evaluate the polarization accuracy by analyzing the relation between calibration error and intensity instability.The work is very important for solar telescopes with high polarization precision up to 10-4 Ic.