Abstract: In-deep investigation and studies are made into the failures of long round thread jump-out and galling of the Φ139.7 mm ×7.72 mm N80 casing as used in one oil well.It is identified through the analysis that the main cause for the said casing thread jump-out is the serious galling of the thread connection as occurred in the make-up process of the casing,during casing running operation,which leads to the connection strength is tremendously reduced.The wellhead center offsets the casing string axis too much,and the coupling is seriously deformed during the make-up process due to clamping with the hydraulic drill pipe jaws.As a result,the matching between the pin and box gravely deformed coupling causes the galling damage of the casing connection.Moreover,the galling of connection is also related to the poor galling-resistance of the casing itself.