The process of wafer polishing is known to be highly demanding, and even small deviations in the processing parameters can have a significant impact on the quality of the wafers obtained. During the process of wafer polishing, maintaining a constant pressure value applied by the polishing head is essential to achieve the desired flatness of the wafer. The accuracy of the downward pressure output by the polishing head is a crucial factor in producing flat wafers. In this paper, the uncertainty component of downward pressure is calculated and its measurement uncertainty is evaluated, and a method for calculating downward pressure uncertainty traceable to international basic unit is established. Therefore, the reliability of double side polishing machine has been sig-nificantly improved.