As an essential component of lifeline systems

water distribution networks play an important role in modern city.Taking network′s annual reduced cost as optimization object and seismic reliability as optimization restriction

a network topology and pipe diameter optimization model is established.The particle swarm algorithm

which takes network as particle and updates the particle’s location to give the optimal network structure

is chosen to solve the optimization problem.Also

a typical water distribution network

which consists of 29 nodes and 47 pipelines

is investigated.Three different network structures are given respectively under three different reliability restrictions.The results show that the approach put forward in this paper are qualified for water distribution networks optimization.