The blowout preventer is closed immediately once the oil or gas well overflows or blows out. During the well shut in period, the blowout preventer will withstand the water hammer pressure generated by shut in. Based on the analysis of the physical characteristics of water hammer propagation in shut in wells, this paper derives the formula for calculating the direct shut in pressure from water hammer. With comprehensive consideration of the factors affecting the water hammer velocity such as gas content in the wellbore fluid, gas liquid two phase flow pattern, solid phase type, the calculation formula of the propagation velocity of the water hammer wave in the three phase mixed fluid of gas, liquid and solid is deduced with the influencing factors of the water hammer wave velocity analyzed through an example. Calculations show that the water hammer wave velocity is very sensitive to changes in the gas content: when the gas content in the drilling fluid is 0～0.001, the water hammer wave velocity changes abruptly. The water hammer wave velocity is only 385m/s when the gas content is 0.001, indicating a decrease by 67% compared with that in the pure drilling fluid; while the solid phase content in the drilling fluid, the inner diameter and the wall thickness of the casing have less influence on the propagation velocity of the water hammer wave.