The photodetached electron flux of H- in magnetic field near a metal surface is studied with a semi-classical open theory, and the relation between the electron flux distribution and classical trajectory is also revealed. The electron flux distributions are calculated at various magnetic field strengths, with a ion-surface distance given. The results show that with the increase of magnetic field strength, the interference pattern in the flux distribution becomes much more complicated because the number of the classical trajectories of the detached electrons contributing to the electron flux distribution increases. In addition, we find that as the energy of detached electron changes, the detached-electron flux distribution changes accordingly. Therefore, the interference pattern in the detached-electron flux distribution can be controlled by adjusting the magnetic field strength and the energy of detached electron. Our study will provide a new understanding of photo-detachment microscopy of anion in external field and surface, and can be used to guide the future experimental research on the anion photo-detachment microscopy.