The recent discovery of a new quantum state of matter, topological insulator, has generated a lot of interest due to its great scientific and technological importance In a topological insulator, spin-orbit coupling opens an energy gap in the bulk, and results in helical surface states. Another essential difference is connected with spin-related properties. In the surface, Hamiltonian of the 3D TI σ acts on the real spin of the charge carriers. Hence, it is natural to manipulate spin transport on the surface of a 3D topological insulator by controlling the electron orbital motion.Based on the topological surface Hamiltonian, it is clear that σ· k is a quantum conserved quantity which implies that spin and momentum of the electron are locked. In a tunneling process, the reflected electron will reverse its spin due to the helical property of the surface states, i.e., the spin-momentum locking This feature will lead to some interesting phenomena, such as the spin-dependent conductance We investigate quantum tunneling through a single electric and/or magnetic barrier on the surface of a three-dimensional topological insulator. Our analysis deal with electron tunneling through single electric and magnetic potential barriers which can be created by depositing a ferromagnetic metallic strip on the surface of a 3D topological insulator. We find that the in-plane spin orientation of the transmitted and the reflected electrons can be rotated over certain angles that are determined by the incident angle and energy. Our results demonstrate that the magnetic field of the magnetic barrier bends the trajectory of the electrons, and therefore rotate the spin.   The low-energy electrons near the Γ point of the Dirac cones can be well described by the effective Hamiltonian   where vF is the Fermi velocity, σi (i = x, y, z) are the Pauli matrices, V is the gate voltage applied on the magnetic metal strips, and the last term HZ ≡ gμBσ · B is induced by Zeeman spin spitting.   First we consider that the incident electrons are spin polarized along the direction of the vector perpendicular to the direction of the electron motion.Note that, perfect transmission always exists in the vicinity of normal incidence, i.e., this is the so-called Klein tunneling, induced by the helical property of the Dirac fermion. This perfect tunneling process can even occur for a low incident energy, and a high and wide barrier.From the spin orientation one can see that the transmitted electron spins are polarized along the same direction as the incident electron spin, indicating that a pure electric barrier without magnetic field will not affect the spin orientation, the reason is that an electric barrier cannot bend the trajectories of the electrons and the outgoing electrons will propagate in the same direction as the incident ones. The spin of the reflected electron, with reversed longitudinal wavevector and conserved transverse wavevector, is rotated due to the spinmomentum locking.   Next we consider the tunneling process through a pure magnetic single barrier.the transmission becomes asymmetric with respect to the in-plane momentum ky parallel along the interface, since the magnetic field breaks the time reversal symmetry. The total reflection is always present regardless of the incidence angle and results in an asymmetric behavior of the transmission as a function of the incident angle. We also examined the electron transmission through a combined electric and magnetic barrier. The interplay between electric barrier and magnetic field strongly reduces the perfect transmission region, and provides us with an additional way to control the transmission and the spin orientation of the transmitted and reflected electrons.