Abstract: In case of overflow or blowout during drilling, the well should be shut down immediately. During the shut-in process, the BOP needs to withstand the shut-in water hammer pressure. This paper analyzes and calculates the shut-in water hammer pressure when the blowout jet is natural gas, drilling fluid and gas-liquid two-phase mixture. Based on this, the shut-in reliability analysis is performed. According to the analysis, only when the blowout jet is drilling fluid and the jet velocity is large, "Soft Shut-in" should be selected to reduce the shut-in water hammer pressure.In other cases, "Hard Shut-in" should be selected to reduce formation fluid intrusion.