The current induced magnetization switching (CIMS) in a nano-scale pseudo-spin-valve (PSV) structure at low external magnetic filed is investigated using a macroscopic phenomenological model based on the magneto-dynamic equation. Magnetization reversal conditions and the corresponding critical currents are obtained by considering both the spin-dependent scattering at the ferromagnetic/nonmagnetic interfaces and the relaxation of spin accumulation in the ferromagnetic layer. The movement of the resistance-current hysteresis of the PSV structure at low external magnetic filed are explained by the calculation. The way to control the crucial current in the CIMS effect with an external magnetic field is also suggested.