Abstract: In the ANANAS Block of the South Sudan, mud acid and bridge plug acidizing technology are adopted in some of the large-size cased wells for blockage removing and stimulation, but the reaction of mud acid is too fast and the bridge string has complicated operation and the risk of acid leak, so it is in urgent need to develop the multi-stage acidizing technology of large-size cased well. A new type of acid injector was developed by designing small-section fixed piston, separating liquid inlet and acid injection port and selecting T-shaped seal ring to seal the acid injection port. In addition, the HV retarded acid system suitable for the reservoirs in the ANANAS Block was selected by means of corrosion inhibition evaluation, complexation testing and compatibility testing. The laboratory tests show that the opening force of the new type of acid injector is up to 8 MPa, which can realize the stable setting of Ø147 mm packer; one-trip string can theoretically realize acidizing of 15 stages and casing leak detection; HV retarded acid system is superior to the conventional mud acid system in term of corrosion inhibition, can effectively complex with reservoir metal ions to inhibit the generation of secondary deposits, and is well compatible with various additives. This technology can be used for the multi-stage (＞3) acidizing stimulation of large-size cased wells and HV retarded acid system can increase the action distance of acid effectively and remove near well damage, presenting remarkable application effects. It is suggested to take the diverting and acid fracturing into consideration to improve the reservoir stimulation degree further.