The invention discloses a converter furnace mouth flange on-line replacement method. The method comprises 11) cutting and separating an old furnace mouth flange from the upper section of a converter shell of a converter and a ribbed plate outside the upper section of the converter shell, 12) cleaning the cutting port of the upper section of the converter shell of the converter body to obtain a butt joint port of the upper section of the converter shell, 13) keeping the butt joint port vertical upward and arranging an alignment tool in an inner hole of a new furnace mouth flange so that the center axis of the alignment tool coincides with the center axis of the new furnace mouth flange, 14) placing the new furnace mouth flange on the butt joint port, 15) hanging down a plumb bob from the very center of an oxygen lance at the furnace mouth and adjusting the top end of the new furnace mouth flange so that the top end of the new furnace mouth flange is perpendicular to the hanging-down direction of the plumb bob and the center axis of the alignment tool is coaxial with the very center of the plumb bob, and 16) welding the bottom end of the new furnace mouth flange to the butt joint port and the ribbed plate. The method solves the problem that the existing furnace mouth flange replacement method using whole converter replacement has high workload and a high replacement cost.