The invention discloses a power distribution network planning project automatic optimization method based on fixed investment, and the method comprises the steps: forming a two-level problem library through machine account library problem identification, wherein the two-level problem library comprises a first-level problem and a second-level problem, the second-level problem is the refinement of the first-level problem, and the second-level problem is divided into three levels, namely a level I, a level II and a level III, according to the severity of the problem; in order to solve the second-level problem, forming a corresponding planning project, adding the planning project into a reserve library, and marking each planning project level according to the problem level; calculating the benefit-cost ratio of each planning project, ranking the III-level planning projects, the II-level planning projects and the I-level planning projects from high to low according to the benefit-cost ratio, ranking the III-level planning projects at the foremost side, and ranking the I-level planning projects at the rearmost side; and accumulating the investment of the top planning project, stopping accumulation after the specified investment is reached, and giving an optimal planning project sequence. The method provides a basis for decision-making personnel, and solves a problem of planning project optimization under the condition of fixed investment of a power supply enterprise.