Abstract: A long round thread casing joint haven been pulled out from factory side of a coupling in a horizontal well, and the dropout casing string couldn′t got out of the well. Research on the dropout failure case was carried out. By means of macroscopic analysis, dimension measurement, thread parameters measurement, physical and chemical properties testing on the material, the full scale tests and mechanical analysis, the dropout cause was analyzed. The results showed that dropout failure was ascribed to the unqualified thread standoff of coupling and the poor thread sealant on anti galling performance.