Structural vulnerability is an electrical term description of a network topology that can remain intact and continues to maintain normal operational capability when the system fails. Recent years, a large number of renewable energy sources are incorporated into the power grid. However, because of the characteristics of output fluctuation, the renewable energy sources will change the structural vulnerability of the power grid when it runs. So it is essential to assess the structural vulnerability of the power grid which contains renewable energy sources. This paper first calculates the overall structural vulnerability index of the grid by calculating the average of the structural vulnerability indexes of each line;then, based on the Monte Carlo method, analyzes the expectation and variance of the overall structural vulnerability index when output fluctuates and guides how to select the sources for power balance adjustment. Finally, choosing the IEEE-39 node system accessing renewable energy source as an example, this paper verifies the validity and feasibility of the proposed method and proves that it has a good guiding significance for the safety and stable operation of the power grid with containing renewable energy sources, and for the prevention of accidents.