Effects on spin-polarized transport properties of interfacial scattering and random substitutional non-magnetic disorder in magnetic tunnel barrier are investigated numerically taking into account spin–orbit (SO) coupling and local s–d exchange interaction under finite bias voltage. The method implemented with calculation of real space Green's function is employed in tight-binding model. Tunnel conductance is analyzed in terms of spin-conserving and spin–flip contributions due to mixing of spin states induced by the SO coupling. Spin polarization can even be inverted due to modification of spin-dependent density of states and shifted Fermi level with bias voltage. Interfacial spin–flip scattering plays a crucial role in the depression of spin-injection rate.