Abstract: The leakage accident of casing string took place during the pressure test after cementing in one well, and then the operation of looking for leakage, injecting cement for block up the leakage, and logging survey for inspection were carried out. The results showed that the casing string seal was failed because of leakage in many casing joint. According to the leakage feature of casing string and the result of pressure test in the laboratory, it was found out that casing string was leaked due to its bad seal performance because of unqualified quality of making up when the casing was running down.