A mobile navigation system includes: a radar antenna carried by the vehicle, and emitting first and second sensing beams at first and second time points, respectively; first and second retro-directive antennas or beam-reflecting objects disposed at first and second positions, respectively, and being a specific distance from each other; and a processing device electrically coupled to the radar antenna. The first and second retro-directive antennas or beam-reflecting objects respectively return first and second retro waves corresponding to a direction of the first sensing beam, and respectively return third and fourth retro waves corresponding to a direction of the second sensing beam. The processing device receives the first, second, third and fourth retro waves, and determines a moving direction of the vehicle according to the first, second, third and fourth retro waves and the specific distance.