Reasonable distribution network planning is an essential prerequisite of the economics and security of the future power grid. The comprehensive benefit evaluation of a distribution network planning project can make significant contributions towards guiding decisions during the planning scheme, the optimization of the distribution network structure, and the rational use of resources. In this paper, in light of the characteristics of the power distribution network, the comprehensive benefit evaluation index system is constructed considering the influencing factors of technical benefit, economic benefit, and social benefit. To eliminate the influence of subjective factors on the evaluation effects and the uncertainty of the influencing factors effectively, the improved interval analytic hierarchy process is employed to calculate the index weights more simply. Moreover, based on the traditional single-factor extension evaluation, this study proposes a multi-level extension assessment model to evaluate the comprehensive benefit of the power distribution network planning project. The model can not only identify the key factors that affect the evaluation effect of the power distribution network planning project, but also can predict the overall development trend of the project. Finally, using a specific urban distribution network planning project as an example, the findings indicate that the comprehensive benefit grade of this power distribution network planning project is “better” due to the benefit grade variable eigenvalue j * ∈ [ 3.33 , 3.418 ] ∈ [ 3 , 4 ] , and illustrates that the model is credible and practical to achieve the comprehensive benefit evaluation of the power distribution network planning project.