The invention discloses a minimum investment calculation method with an optimal planning effect. The method comprises the steps of determining a planning index and obtaining a current situation value;determining a target value and a comprehensive weight value of the planning index; establishing a mapping relationship between the planning indexes and the effects of the to-be-built projects, and dynamically adjusting the effect weight of each to-be-built project; carrying out normalization scoring on all to-be-built projects; calculating a comprehensive effect score of each to-be-built project;and determining the minimum investment demand according to the minimum investment model with the optimal effect. According to the method, the current situation data of the to-be-built project and thepower grid are comprehensively considered, and a dynamic adjustment mechanism adapting to the project score and the planning target index weight is established, so that the optimal effect of the investment amount in different types of projects and different voltage levels can be realized, and the method is high in reliability and high in accuracy.