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We have investigated the transport properties of the Dirac fermions through a ferromagnetic barrier junction on the surface of a strong topological insulator. The current-voltage characteristic curve and the tunneling conductance are calculated theoretically. Two interesting transport features are predicted: observable negative differential conductances and linear conductances tunable from unit to nearly zero. These features can be magnetically manipulated simply by changing the spacial orientation of the magnetization. Our results may contribute to the development of high-speed switching and functional applications or electrically controlled magnetization switching.