Abstract: An investigation is conducted into the connection pull over failure of the casing stage cementing collar as used in a certain domestic oil well，and it was found that the measured parameters of the external threaded connection of thestage cementing collar and the material is in compliance with the requirements of relevant standard. Furthermore a systematic trouble-shooing and analysis are carried out to all the operation steps in association with the casing and the stage cementing collar，including transportation，storage，inspection and usage，based on which，it is regarded that the reason for the connection pull over failure is the non proper make up of the threaded connection of the stage cementing collar due to the deformation of the casing coupling. Accordingly it is suggested to strictly implement relevant customer company’s standard，and also perform stringent inspection and evaluation of the casing protection ring.