To reflect the components' real working process during the lifespan and its impact on the reliability of power distribution system, the concept of bathtub-shaped hazard function is applied to components' reliability modeling. It is more agreeable with the engineering practice to use the modified Weibull distribution and the logarithmic normal distribution to describe the lifespan and the time to repair respectively. The sequence operation theory is applied to calculate the probability distributions and the expectations of power distribution system reliability index, thus overcoming the defect of traditional analytical method that cannot deal with the non-exponential distribution system, and providing a new approach to reliability evaluation. The effectiveness of the proposed method is verified by a case study of RBTS-BUS6 system.