Abstract: The paper gives a description of the idea of selecting the conjugate prior distribution using Bayesian method, and makes a detailed discussion on the technique of converting the Lognormal distribution to Gamma and Beta distribution by pivot conversion when the prior distribution of running failure and demand failure are abided by Lognormal distribution, which bears the characteristics of simplicity and convenience in calculation, as well as being easy to engineering application. Furthermore, it serves as the foundation for developing the software suitable of calculating the reliability parameters for nuclear power plant.