Parameter randomness has an important influence on the safety of penstocks of hydroelectric station, but the reliability analysis of the penstocks at present is mainly based on the component reliability of the primary parts, having no full reflection of the integral safety of the penstocks from the system reliability point of view. In this paper, a series system consisting of four classical parts is proposed by considering the correlation between basic parts, and the system reliability of the typical engineering examples of the exposed penstocks is analyzed to identify a part which will control the system failure using the second-order bounds based system reliability method. Then analysis of the impact of random parameters upon the system reliability is carried out. The analysis results suggest that the supporting ring and nearby pipe wall are the control parts of the system failure of the exposed penstocks designed in terms of the present specifications. The impact of mean values of the random parameters on the system reliability is significant, so the parameter fluctuation range should be controlled strictly in structural design, construction and operation to improve the integral safety of penstocks.