Spin dephasing in organic semiconductor was studied based on spin drift-diffusion model in various electric field. It is found that in the ohmic regime, spin dephasing is determined by the voltage bias. With increase of the voltage bias, spin coherence increases. And the increasing voltage bias can enhance the magnitude of the spin dephasing oscillations at a static perpendicular maganetic field. It shows that the current density in total device output can be modulated by an electrostatic field via controlled precession.