A warning method and system for detecting lane-changing condition of rear-approaching vehicles are disclosed, in which the method comprises the steps of: detecting rear-view images of an ego-vehicle so as to be used for defining three regions of interest (ROI) corresponding respectively to a lane to the left of the ego-vehicle, a lane to the right of the ego-vehicle, and a lane where the ego-vehicle is travelling thereon; detecting whether there is any direction light that is flashing to be used as basis for determining whether there is any vehicle approaching from the rear that is expected to be a potential threat to the ego-vehicle; and if so, issuing an alarm signal to the driver of the ego-vehicle for reminding the same to aware the distance between the ego-vehicle and the rear-approaching vehicle. Thereby, the driver's road environment awareness is enhanced and consequently the safety of driving can be improved.