MnZn ferrite, characterized by the low power loss, the high electrical resistivity, and the high magnetic permeability, is a promising candidate for the low-noise magnetic shielding applications. However, as the dissipative material, MnZn ferrites inherently generate a magnetic noise, which fundamentally limits the sensitivity of the magnetometers operating in the spin-exchange relaxation-free (SERF) regime. Herein, we report a synergistic co-doping strategy using Co₂O₃ and Bi₂O₃ to effectively suppress the intrinsic magnetic noise of MnZn ferrites. By systematically tuning the concentrations of Co₂O₃ and Bi₂O₃, the power loss, complex magnetic permeability, and microstructural evolution of undoped, singly doped, and co-doped MnZn ferrites are comprehensively investigated. Notably, co-doping with 1600 ppm Co₂O₃ and 400 ppm Bi₂O₃ reduces the low-frequency magnetic noise by more than 50%, which is attributed to refined grain boundary structures and suppressed hysteresis losses. The optimized MnZn ferrite is further employed to fabricate a magnetic shield for a SERF magnetometer, achieving a single-channel sensitivity of 0.25 fT·Hz^-1/2. This study paves an effective materials-engineering route to minimize magnetic noise in ferrite-based shielding systems, providing a solid foundation for the development of next-generation ultra low-noise platforms for the quantum precision measurements.