Abstract: Traditional distribution network planning method is to minimize the cost of the power network based on certain reliability．The purpose of the planning is to ensure power system reliability，so the benefit derived from the expansion of power network is ignored，thus the optimized social benefit can not be acquired．Through the introduction of outage cost combining the reliability and economy，a new mathematical model and cost-benefit analysis method of power network planning is presented in this paper． The method takes the outage cost as a part of the total cost and quantifies the reliability benefits by the outage cost，and the costs and benefits about power network reliability planning can be evaluated based on a consistence of economy．Through the analysis and evaluation of a planning for a concrete power system，the application of cost-benefit analysis in power network planning is further improved． The efficiency of this method is validated in some test examples．