Abstract: The valve side winding of converter transformer has to bear DC electric field perennially, which introduces severe discharge to its insulation due to cellulose impurity bridge. For this reason, the bridging and discharging characteristics of cellulose impurities in oil-paper insulation under DC voltage is systematically stated. Firstly, the generation mechanism of the impurities under electric, thermal and flow fields is discussed, and it is shown that the effect of space charge and electric field force under electric field, thermal cracking and oxidative cracking under thermal field, and molecular vibration under flow field may lead to cellulose shedding. Secondly, the migration law of free cellulose impurities is discussed. It is proposed that the movement of the impurities is mainly influenced by gravity, buoyancy, Coulomb force, dielectrophoresis force, and viscous resistance, whereas DC electric field is a necessity for bridging. Afterwards, the mechanism of oil discharge induced by the impurities under DC voltage is introduced, and it is pointed out that cellulose impurities can form a "core-shell" conductive structure with water, and can cause discharges based on the field distortion effect and metastable liquid nucleation/vaporization effect. On the basis above, key scientific issues are proposed, including exploring the molecular chain breaking mechanism of cellulose impurities under the synergistic effect of electric, thermal and flow field, and the influence of cellulose impurity's kinetic behavior on charge behavior in oil is revealed.