A precision method for the reliability cost-benefit analysis is proposed by analyzing the cost and benefit of distribution network reliability. By decomposing the marginal reliability cost-benefit curve into different curves， the most economical and rational scheme is obtained for targeting a certain reliability value. The proposed precision analysis method is applied to evaluating and optimizing the distribution network planning for four typical areas in Nanjing during the Twelfth Five-Year Plan. Adoption of the optimal scheme obtained has yielded obvious economic benefits.