Electric field tailored magnetic properties of the perovskite-type oxide heterostructures are important in spintronic devices with low energy consumption and small size. Here, the electric field modulated magnetic properties of underoxidized SrRuO₃ (SRO)/SrTiO₃ (STO) heterostructures are investigated using first-principles calculations. The spin polarization of underoxidized SRO/STO heterostructures turns from negative to positive as the electric field changes from −0.2 to 0.2 V nm⁻¹. The underoxidized SRO/STO heterostructure with 7 SRO atomic layers turns from perpendicular magnetic anisotropy to in-plane magnetic anisotropy as the electric field turns from −0.2 to 0.2 V nm⁻¹, which can be attributed to the in-plane d_x²–y² and out-of-plane d_xz, d_yz orbitals. The Dzyaloshinskii–Moriya interaction of underoxidized SRO/STO heterostructures can also be effectively tailored using an electric field. These results indicate that the use of electric field is an effective method to modulate magnetic properties of perovskite-type oxide heterostructures, which is beneficial for the development of the high-performance spintronic devices.