Abstract: The Laser Interferometer Space Antenna （LISA） mission requires the residual acceleration noise of the test mass （TM） to be below 3×10⁻¹⁵ m·s⁻²·Hz^−1/2, with magnetic noise being a major disturbance source. This study investigates the magnetization characteristics of magnetic moment sources, mainly the electrode housing （EH） and negative temperature coefficient thermistors （NTCs）, located within the millimeter-scale near-field region of the TM. A high-fidelity multi-physics simulation model is established to quantify the full-chain transmission pathway from magnetic moments to field characteristics and finally to magnetic noise. The key findings are as follows: （1） The magnetization state of the NTCs dominates the local magnetic gradient. Fully magnetized NTCs produce an average gradient of 2.76×10⁻⁵ T/m at the TM, exceeding the design gradient requirement by a factor of 27, and leading to a magnetic acceleration noise of 3.19×10⁻¹⁵ m·s⁻²·Hz^−1/2. After demagnetization, the gradient is reduced to 1.61×10⁻⁶ T/m, and the noise is suppressed to 6.21×10⁻¹⁶ m·s⁻²·Hz^−1/2, meeting the allocated requirement of 1.21×10⁻¹⁵ m·s⁻²·Hz^−1/2; （2） Constraining the residual magnetic moment of the EH to ≤80 nA·m² reduces its contribution to the static magnetic field at the TM from 23.6% to 5.66%, and decreases the peak field at edges by 63.5%, significantly improving field uniformity; （3） A vector coupling effect is revealed between fluctuations in the interplanetary magnetic field and the static field generated by the NTCs, amplifying the total magnetic fluctuations by 16.4%. Demagnetization effectively mitigates this interference by reducing the static component. The study validates the effectiveness of demagnetization in suppressing ultra-low frequency magnetic acceleration noise and provides a theoretical foundation for the magnetic cleanliness design of future space-based gravitational wave detection missions.