1. A pre-warning method, utilized in a vehicle radar system comprising a first millimeter-wave detection module and a second millimeter-wave detection module, the pre-warning method comprising: detecting dynamic information of a plurality of first targets in a first dimension within an area corresponding to the vehicle radar system, to obtain a first detection result; detecting dynamic information of a plurality of second targets in a second dimension within the area corresponding to the vehicle radar system, to obtain a second detection result, wherein the first dimension is different from the second dimension; determining whether there are identical targets within the plurality of first targets and the plurality of second targets according to the first detection result and the second detection result; after determining that the plurality of first targets and the plurality of second targets comprise at least an identical target, determining a 3-dimensional (3-D) dynamic information of the at least an identical target; and determining whether to trigger an alarm signal according to the 3-D dynamic information of the at least an identical target.