Abstract: After a well kick or well blowout occurs, shutting down the blowout preventer (BOP) is an important measure to protect the wellbore and avoid the further deterioration of the accident. It is of great practical significance to determine the shut-in load and delimit the shut-in time for developing and operating high-pressure wellhead equipment. Against the background of domestic development of ultra-highly BOP, in view of an extremely harsh condition, which means that during a blowout/well surge, a large amount of formation gas kicks into wellbore and the drilling fluid is pushed upward faster and faster, mathematical models that describe water hammer wave travelling in the annular space after hard shut-in are established by means of transient flow theory. Extreme loads that the BOP is subjected to are predicted when the hard shut-in happens at different times after massive gas invasion. It provides a basis for designing, selecting and checking wellhead device. The shut-in time is recommended according to the pressure level of the BOP. It is proposed that a monitoring device for return rate of drilling fluid should be installed at wellhead. The well could be shut in automatically when the velocity reaches the warning value, and the safety of the wellhead could be ensured.