A reliability indicator called the network risk index capable of reflecting the rationality of the distribution network structure is proposed.By taking the investment cost，network loss and network risk index as the objective functions，a distribution network planning model is developed with an overall consideration of the economy and reliability of the distribution network.A network repair algorithm based on bus incidence matrix is designed to find and repair the randomly generated isolated busses，isolated chains and closed loops for the requirement of grid radiation.New fitness functions are proposed to improve the search speed and capability of the strength Pareto evolutionary algorithm（SPEA）.To sort the individuals in the Pareto frontier，the technique for order preference by similarity to an ideal solution（TOPSIS）is used and the optimal solution is screened out.Finally，the model is used in a 54-bus network planning.The result shows that there is a strong linear correlation between the network risk index and the outage loss，thus the network risk index can be used as an indicator of distribution network reliability.Compared with the traditional SPEA，the improved algorithm has a better search ability and a faster search speed.