The mechanism of terahertz (THz) pulse generation with a static magnetic field imposed on a gas plasma is theoretically investigated.The investigation demonstrates that the static magnetic field alters the electron motion during the optical field ionization of gas,leading to a two-dimensional asymmetric acceleration process of the ionized electrons.Simulation results reveal that elliptically or circularly polarized broadband THz radiation can be generated with an exteal static magnetic field imposed along the propagation direction of the two-color laser,The polarization of the THz radiation can be tuned by the strength of the exteal static magnetic field.