Customizing the frequency range of electromagnetic wave (EMW) absorbing materials, especially for low-frequency, is a key research focus for 5G/6G and stealth applications. However, achieving precise low-frequency tuning remains challenging due to unpredictable parameter variations in practical design. Here, a constant-permeability-based electromagnetic parameter inversion method predicts the required complex permittivity range for multilayer MXene’s effective microwave absorption in the target low-frequency band. Since traditional modulation methods are plagued by electromagnetic parameter fluctuations, this study regulated the dielectric response by adjusting the embedding amount of small-sized iron nanoparticles (Fe NPs) with stable permeability. Under this guidance, multilayer MXene/Fe NPs (MTF) are prepared by embedding small-sized Fe NPs on the MXene surface via electrostatic self-assembly and in-situ reduction. The introduction of Fe NPs increased charge carriers’ concentration and strengthened the interface effect, resulting in a significant increase in the real part of the complex permittivity (ε') compared with that of multi-layer MXene (7.13–8.89), reaching the predicted range of the real part of the low-frequency complex permittivity (13.12–15.16, 14.34–16.81, and 15.29–18.12). Experimental results show that the MTF has a small error in the frequency of the minimum reflection loss (RL_min) compared to the predicted value (error percentage of 4.69%), along with an in-situ enhancement of the effective absorption bandwidth (EAB) (325.00% growth). Thus, MTF exhibits enhanced low-frequency absorption, with MTF-2 achieving −46.3 dB RLmin at 4.64 GHz (4.35 mm) and 2.24 GHz EAB at 3.8 mm. This work offers a strategy for accurate prediction and regulation of absorption bands over a wide range.