1. A pre-warning method, utilized in a vehicle radar system comprising a first millimeter-wave detection module, a second millimeter-wave detection module and a plurality of radiating units, wherein the plurality of radiating units form a plurality of antenna strings, radiating units of each antenna string are connected in a sequence, and the plurality of antenna strings are classified into a first group and a second group, the pre-warning method comprising: the first millimeter-wave detection module and antenna strings of the first group within the plurality of antenna strings radiating in a horizontal polarization to detect dynamic information of a plurality of first targets in an azimuth dimension within an area corresponding to the vehicle radar system, to obtain a first detection result, wherein the azimuth dimension has a first direction, and when the first millimeter-wave detection module transmits or receives RF signals to detect the dynamic information of the plurality of first targets via the antenna strings of the first group, a first electric field generated by the antenna strings of the first group extends toward the first direction; the second millimeter-wave detection module and antenna strings of the second group within the plurality of antenna strings radiating in a vertical polarization to detect dynamic information of a plurality of second targets in an elevation dimension within the area corresponding to the vehicle radar system, to obtain a second detection result, wherein the azimuth dimension is orthogonal to the elevation dimension, the elevation dimension has a second direction, and when the second millimeter-wave detection module transmits or receives RF signals to detect the dynamic information of the plurality of second targets via the antenna strings of the second group, a second electric field generated by the antenna strings of the second group extends toward the second direction; 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; wherein at least an antenna string of the plurality of antenna strings belongs to both the first group and the second group.