On the basis of the tight-binding Su-Schrieffer-Heeger model， the dynamical process of charge injection from metal electrode to a polymer in a metal/poly(p-phenylene vinylene) (PPV)/metal structure has been investigated by using a nonadiabatic dynamic approach. It is found that the injected charges form wave packets due to the strong electron-lattice interaction in PPV. The formation of the wave packet sensitively depends on the applied voltage， the electric field， and the contact between PPV and the metal electrode. In the absence of external fields， the charge can be injected into PPV chain and form wave packet under the critical bias voltage. With the increase of the electric field， the charge carriers can be ejected from PPV to the right metal electrode. But when the electric field becomes high enough， the wave packet can not be formed because it moves too fast in PPV. It is also shown that the number of ejected charges is not an integral multiple of the elementary charge e.