The dynamics of the magnetization reversal process in permalloy films by the pure spin current injection has been analyzed. We have performed micromagnetics simulations based on the Landau-Lifshitz equation taking into account both the relaxation term and spin transfer torque term caused by the pure spin current which is injected into the small permalloy thin films laterally. We found that the magnetization switching time is decreased and the switching probability is increased by optimizing the spin polarization direction of the injecting pure spin current.