In this paper, a low-power high-response wireless structural health monitoring system (WSHMS) is designed, implemented and experimentally evaluated for impact detection in composite airframes. Due to the rare, random and transitory nature of impacts, an event-triggered mechanism is adopted for allowing the system to exhibit low power consumption when no impact occurs and high performance when triggered. System responsiveness, robustness and energy efficiency are considered and modelled. Based on system requirements and functions, several modules are proposed, including filtering, impact detecting, local processing and wireless communicating modules. The filtering module increases the system robustness by attenuating background vibration noises. The impact detection module monitors impact categories, and when the impact energy is above a certain threshold, it generates a trigger (wake-up) signal for the local processing module. The local processing module is required to be responsive to impact events, capable of processing multiple sensing inputs and energy-efficient when no impact occurs. The wireless module transmits the processed data to the host station for impact evaluation. The whole design was implemented on a printed circuit board (100 × 65 mm). The response time is around 12 µs with an average current consumption lower than 1 mA when the impact activity is lower than 0.1%. The system exhibits high robustness to ambient vibration noises and is also capable of accurately and responsively capturing multiple sensing input channels (up to 24 channels). This work presents a lowlatency energy-aware WSHMS for impact detection of composite structures. It can be adapted to monitor of other rare, random and ephemeral events in many Internet of Things applications.