The invention provides an online repairing method for leakage of an O-shaped ring of a large-size flange. According to the online repairing method, a flange, a filling tool, a double-component binding material, a mechanical pump, a high-pressure hose, a priming coat agent and a sealed cavity are mainly arranged. Shown as the figure, the technical scheme provided by the invention is as follows: one side of the flange is connected with a double-cavity injection gun for containing the double-component binding material and the other side of the flange is connected with the mechanical pump through the high-pressure hose; upper and lower slits of the whole flange are coated with the priming coat agent and the sealed cavity is arranged outside the flange. According to the online repairing method, one sealed cavity is established at a leakage part of the flange and thrust greater than pressure of a medium system is adopted to inject a sealant which has good flowability and curing speed, has good base material binding performance and is easy to remove to fully fill the sealed cavity, so that a leakage hole of the O-shaped ring is blocked; the sealant can be rapidly cured under a certain condition so that the leakage is stopped; a new and reliable sealing structure is established and the aim of eliminating the leakage is realized.