A macroscopic phenomenological model based on the magnetic dynamic equation was proposed to investigate spin-polarized current induced magnetization switching in a nano-scale pseudo-spin-valve structure. The movement and even reversal of the free layer's magnetization resulted from two factors, the net spin flux flowing into the free layer and the spin relaxation within it. The dynamic equation incorporated the spin-dependent scattering at the ferromagnetics/nonmagnetics interfaces, and the relaxation of spin accumulation in the ferromagnetic layer. Conditions of magnetization reversal and the corresponding critical currents were found by solving the dynamic equation analytically. The qualitative and quantitative results agree with most of experiments reported. According to the model, the influencing factors on the critical current were analyzed and the methods to improve device performances were pointed out.