This paper designs and improves power efficiency and integrated power efficiency evaluation index, and the influence on the power supply efficiency of distributed generation directly access to the main power grid is analysised by using complex network theory. The vulnerability assessment under different fault mode of power grid containing distributed generation is done based on an integrated power supply efficiency index, then, the IEEE39 nodes system is tested ,and comparatively analysis the power grid vulnerability at distributed generation to join before and after under different fault modes in order to verify the validity of indicators. The qualitative assessment the impact on the grid vulnerability at distributed generation to join before and after has important theoretical and practical significance to develop and apply clean energy.