Abstract: Geomagnetic polarity reversal was often happened in the geologic history. The author tries to probe its mechanism. In the paper, Earth sinterior is divided into five dynamics layers: lithosphere, rheosphere, mesosphere, liquid sphere and solid nucleus. It is considered that the mesosphere and solid nucleus may asynchronously rotate, and this motion drives closed-vortexes in the liquid sphere. Earth magnetic dipole field originates from each other effect between Galaxy magnetic field and the closed-vortexes in Earth's liquid sphere.The geom agnetic polarity depends on the directions of Galaxy magnetic field and the closed-vortexes in the liquid sphere.While one of the two changes its direction,geomagnetic field changes its polarity.