There are so many large-scale wind farms, that detailed modeling and SSO risk analysis for each wind farm one by one will face the difficulty of wind farm modeling, parameter acquisition and longer time spending. The system strength is determined by calculating the Short-Circuit Ratio (SCR) of grid-connected system, which is used to evaluate the SSO risk of wind farm. Analysis based on conventional SCR formula shows that the method can only consider the short-circuit capacity and wind farm capacity at the grid-connected point, and it is difficult to accurately reflect the SSO risk of large-scale wind farm. A calculation method of Aggregate Short-Circuit Ratio (ASCR) is proposed, and the corresponding formula is given. The calculation method can reflect the influence of the factors on SSO, including the grid structure of wind farm, the interaction of sub-wind farms and the control system. Through analyzing the typical and practical system examples, the ASCR can correctly calculate the SSO risk of wind farm for different control systems, and rank the SSO risk of four actual wind farms with different topological structures, which verifies the effectiveness of the method on risk screening of wind farm SSO. This work is supported by National Key Research and Development Program of China (No. 2017YFB0902004), National Natural Science Foundation of China (No. 51725702), “111” Project of China (No. B08013), and Science and Technology Project of State Grid Corporation of China (No. 529926160002).