A radar detection device is provided. The radar detection device may include a radar device and a signal processing device. The radar device may include a plurality of antennas to transmit and receive radar signals. The signal processing device is coupled to the radar device, and the signal processing device determines that a target object is in a first area of a detection area or in a second area of the detection area according to the radar signals received by the radar device to determine to perform an activity detection or a vital-sign detection on the target object.