It is difficult to use the “N-1” criterion to reasonably measure the optimal load capacity of the line under different load structures, and failed to fully consider the economics of line operation under different load structures. A method for evaluating the optimal operational efficiency of medium-voltage distribution lines considering reliability and economy is proposed. Firstly, combining the safety and economy, the operational efficiency of the medium voltage distribution line is defined. Secondly, based on the analysis of the relationship between the reliability index and the line load rate, the reliability evaluation process of the medium voltage distribution line with different load structures is proposed. Thirdly, a model for the optimal load capacity of medium-voltage distribution lines is constructed with the goal of optimal economy. Finally, an example is analyzed to verify the effectiveness and practicability of the proposed method.