Spin-polarized transport in magnetic pn junction has been theoretically studied by using the drift-diffusive theory. The factors considered include bias voltage, equilibrium spin polarization,spin injection and spin life time, which could effectively affect the current density and resistance. The connection between spin-voltaic effect of the magnetic pn junction and thickness of the junction was also discussed. It is found that the equilibrium spin polarization makes the spin electrons with different orientations (up or down) have different barriers, which makes the current effectively modulated. However, the spin injection modulates the current by providing non-equilibrium spin electrons in the magnetic pn junction. It is also found that the spin-voltaic current is sensitive to quasi-neutral p-type region width.