Augmented Reality (AR) technology has emerged as a promising tool for enhancing safety in the construction industry by improving the Situation Awareness (SA) of onsite workers. However, there is a lack of methods to quantify the impact of real-time AR visual warnings on developing and updating SA. To address this gap, this paper presents an eye-tracking-based method that quantifies the impact using three metrics: Time to First Fixation (TFF), Dwell Time (DT), and Revisit Interval (RI). A quasi-onsite experiment validated the feasibility of the proposed method and confirmed that AR warnings could reduce the time to reach Level 1 SA by 40.7% and reduce the interval for updating Level 2 and Level 3 SA by 43.45%. This method can be further combined with advanced visualisation measures to address perception, memory, thinking and mobility, eventually leading to insights on mental effort, decision making, interaction with computers, human reliability and work stress.