Abstract: Sidetracking technology has become one of the main means to develop potentiality remaining oil of interlamination, but the problem of casing leakage has gradually been becoming one of restricting factor in the development of sidetracking wells. There are three types of casing leakage including top casing leakage, extension hanger leakage and sidetracking casing leakage/outside sidetracking casing channeling. The major reasons for different leakages of sidetracking wells were analyzed in this paper. Furthermore, the leakage sealing and plugging technology for different leakages was presented together with typical well examples, which obtained ideal result in field practice.