Abstract: The φ177.8 mm casing with buttress thread connection in an oil well had a pulling out accident. Found out the causes of casing pulling out through by analysis on the process of casing running, cementing, drilling cementation plug and physical testing and chemical analysis of casing remains from well bottom. The results show that pulling out of casing joint from mill end occurred during cement solidification. The casing loosed under abnormal load because of severe dogleg severity of kick-off section, borehole irregularity and no pocket under casing guide, which finally led to the polling out of casing.