Abstract: PMIA paper is an ideal substitute for cellulose paper because of its excellent insulation strength and heat resistance. We compared the breakdown strengths of laminated PMIA paper and cellulose paper in extremely inhomogeneous electric field in order to study the substitutability of PMIA paper for cellulose paper under dry working conditions, and carried out partial discharge tests under step-up and constant voltage for laminated PMIA paper and cellulose paper. The results show that the dry breakdown voltage of the PMIA paper in the extremely inhomogeneous electric field is about twice that of the cellulose paper, and the difference of the breakdown strength between the two becomes smaller with the increase of the number of layers. In the step-up partial discharge, the discharge characteristics of the two insulating papers are similar when the electric field strength is lower than 50% of the respective breakdown strength of the two insulating papers, and after further increasing the electric field strength, the PMIA paper shows a more intense discharge along the surface, and the discharge is about 6 times that of the cellulose paper. In the constant-voltage partial discharge, the discharge of PMIA paper is more intense at the beginning of the discharge phase, with the maximum discharge varying between 6 500 and 43 000 pC, then fluctuating slightly above and below 6 500 pC, and the maximum discharge increases dramatically to 49 966.9 pC near the breakdown. What’s more, partial discharge causes the PMIA paper to show a more severe degree of paper defects, which results from the breakdown of the interface between the short-cut fiber and the pulp.