To investigate the dynamic characteristics of an ultra-supercritical circulating fluidized bed (CFB) boiler during main fuel trip (MFT) and boiler trip (BT) events, a mathematical model of a 660MW ultra-supercritical CFB boiler was established using the Apros simulation platform. The model dynamically simulates MFT and BT scenarios under coal conbustion conditions at 100% BMCR (boiler maximum continuous rating), 75% THA (turbine heat acceptance) and 50% THA operating loads. Numerical results show that under 50% THA operating conditions, the steam-water separator rapidly transitions from dry to wet operation following MFT and BT activation and gradually returns to dry operation after system reset. These frequent dry-wet transitions increase thermal stress on the steam-water separator, potentially reducing its service life and compromising equipment safety. Therefore, triggering MFT and BT under low-load conditions should be avoided to ensure system reliability.