The work aims to study PP cotton filter element on its performance of purifying offset printing dampening solution and select the suitable filtration stage for offset dampening solution filter system under the premise of meeting the filtering requirements. The 4-stage dampening solution filtration and purification system was set up to measure the pH value, conductivity and surface tension of the dampening solution after each stage of filtration, with PP cotton filter element (bore diameter: 0.5 and 1 μm) as filtration medium. PP cotton filter element with bore diameter of 1 μm was regarded as the filter element at stage 1, PP cotton filter element with bore diameter of 0.5 μm was the filter element at stages 2, 3 and 4. After filtration of stage 4, the core parameters of dampening solution were within the range required by printing, but the rate of change was less than 1%. The optimal filtration stage of such offset dampening solution filter device is the filtration at stage 3.