Abstract: For the thread-tripping failure of the Φ88.9 mm×6.45 mm P110 NU tubing as used in a certain oil well, an in-depth investigation is conducted, involving such activities as macro/micro morphology analysis of the failing tubing, physicochemical testing of the material, parameter measurement of the tubing connectors which are of the same product lot, but not yet used. Based on the testing and research results, it is identified that since both the material and the thread parameters of the tubing connector are up to applicable specification, the pull-out failure is defined as a low-load thread stripping case; and that the main causes for such a failure are thread galling and inappropriate thread making-up of the connector, which result in tremendous reduction of the connection strength; and what caused the thread galling and inappropriate thread making-up mistakes is just thread alternating as occurred during the making-up process.