Aiming at the casing integrity after long period of production, as for the traditional remedial measures, such as with straddle packer, cannot meet the post-operation requirements for borehole size, external casing patch technology was proposed. The paper introduces the principles of the external casing patch and the application in HZ19-3-6 and HZ26-1-16 of Huizhou oilfield. The results show that the repaired casing can maintain the original strength and produce permanent seal with the advantages of remaining borehole size. The analysis is made on the complicated situations in operation process, which has practical and theoretical guiding significance for the repair work of marine oil casing integrity and provide reference and inspiration to casing repair of water well and geothermal well.