Aiming at the need of revising the power industry standard in China："Design Code for Steel Penstocks of Hydropower Stations (SD 144-85)", in accordance with the theory of structural reliability, some researches on the statistical characteristics of water hammer pressure in penstocks of hydropower stations have been done. Based on measured data collected from partial foreign and domestic hydropower stations, using the method of hypothesis testing of statistics, stochastic analysis on water hammer pressure is conducted and an optimum probability distribution function and its parameters are obtained. Research shows that the probability distribution model of relative water hammer pressure may be simulated by the extremum type-Ⅰ distribution. These results not only provide necessary basic data for the reliability analysis of penstocks in hydropower stations, but also have important application values for appraisement and evaluation of the reliability of built penstocks in hydropower stations. The influence of water hammer pressure to the reliability of hydro-penstocks is also discussed through an example of exposed penstock reliability analysis, and some important conclusions, which may be referenced in the design of hydro penstocks, are gained.