Strong grid structure can significantly reduce the scope and time if power outage of distribution system, improve the reliability of power supply. The main measures to improve the grid structure mainly include the retrofitting and optimization planning of the automation switches. This paper proposes a method for evaluating weak lines in distribution network, which provides a theoretical basis for searching of weak lines of power grid structure. The reliability improvement index is proposed to evaluate the power grid structure from the perspective of power supply reliability. In order to calculate the reliability improvement, an improved harmonic search swarm algorithm is proposed to obtain the switch distribution scenario with the greatest reliability improvement. This paper proposes a comprehensive evaluation method based on entropy-fuzzy-VIKOR to comprehensively evaluate the reliability of the medium voltage grid structure. 30 lines are selected in a certain area for example analysis, the results show that the proposed method can effectively screen out the weak lines of grid structure and realize the accurate positioning of the switch optimization planning project.