The rubber core seal failure of the spherical blowout preventer(BOP) is very likely to occur during forced tripping operations.In order to study the failure mechanism of the spherical rubber core in the process of dynamic sealing,the numerical model of dynamic seal of spherical BOP was established based on the theory of large deformation of rubber.The stress distribution and failure mode of spherical BOP core during sealing and tripping were studied. The results show that in the process of sealing drill pipe,the inner wall of rubber core will produce strip fold,and a high stress distribution in the shape of a transverse drum is produced at the corner of the plate under the support bar, and there is stress concentration in the neck and back of the support bar.In the process of sealing inclined drill pipe,the sealing area on the drill pipe is inclined,and the stress value on the left and right sides of the rubber core increases slightly.In the process of tripping,the strip-type stress concentration zone in the inner wall of the rubber core changes to the oval stress concentration zone.When the drill pipe joint enters and exits the sealed area,the contact stress will change abruptly.