The interface of perovskite oxides has recently become a platform to functionalize novel physical properties, both theoretically predicted and experimentally examined, benefiting from the advances of lay-by-lay film growth technology. In this work, by the combination of both surface polar discontinuity and interfacial ferroelectric (FE) polarization discontinuity at the BiFeO3 (BFO)/LaFeO3 (LFO) heterostructures (HSs), we can not only realize magnetic ordering and 2D electron gas but also tune these properties from insulating antiferromagnetic ordering to 100% spin-polarized half-metallic ferromagnetism by reversing the polarization direction of BFO. Density functional theory calculations uncover the mechanisms of the polarization manipulation, that is, the exchange interaction of interfacial magnetism occurs alternately between double-exchange and superexchange accompanied by changing the rotation angle of the in-plane octahedron and modification of the electron number at the interface induced by FE polarization reversal. In addition, considering the possibility of sharp phase transition happening for low-dimension systems, the HS with the FeO2 terminal and with three LaO layers has been proved to possess an optimum structure, showing the largest magnetic moment after calculation comparison by changing the HS terminal and the thickness of LFO. Our work will help to fabricate novel devices for information memory or sensors based on electric field tuning of magnetic and transport properties.