The concept of Internet of Things (IoT) has incubated a whole generation of smart applications to resolve problems in society. Despite cloud-based IoT systems inheriting the robustness and scalability of cloud computing, its high latency limits the implementation of time-sensitive applications. To encounter this, Edge-computing-based IoT systems make computing services closer to users which entails lower latencies. New technologies bring new attacks, and Distributed Denial of Service (DDoS) attack has been regarded as one of the major threats to Edge Internet of Things (EIoT) systems. Previous works often focus on the state-of-the-art and the defense mechanism of edge computing. However, there lacks a general standardized method for the security impact analysis, dedicated to EIoT. This paper proposes the DDoS Impact Analysis Index, or DIADex, which is compatible with the IEEE P2668 standard. From the aspects of performance and resources, this method quantifies the impact of DDoS attacks on EIoT systems with a scoring system, which can be used for evaluation in future research and penetration test on EIoT.