A halo plasma, which is a low density plasma surrounding field reversed configurations (FRC), was observed in the Staged Theta Pinch-Linear experiment. As long as the plasma is kept free from the end walls with the help of a guide field structure, azimuthal ion flow builds up in the halo. The direction of the ion flow at the edge of the halo is opposite to that in the vicinity of the separatrix. This double structure of the ion flow can be explained by a model which assumes that the halo plasma is formed by energetic ions shuttling between the FRC and the open field region. The numerical results are in good agreement with the experiment. The model also suggests that the FRC has a good confinement ability for energetic particles if the plasma is not in contact with the discharge tube.