The remarkable advantages of heterojunction engineering have injected significant vitality into the design of high-performance electromagnetic wave absorption (EWA) materials. Understanding interface effects, rather than semi-empirical rules, can facilitate the rational design of heterostructures, thereby enabling effective modulation of impedance matching and the EWA properties of materials. Herein, FeTe@expanded graphite (FeTe@EG) heterostructures are in-situ constructed via a one-step chemical vapor deposition (CVD) method, which effectively generates abundant Mott–Schottky heterojunctions and exhibit a strong built-in electric field (BIEF) effect. The optimal sample, featuring only 10 wt.% filler content and a thickness of 1.8 mm, achieved an effective absorption bandwidth (EAB) of 4.6 GHz and a minimum reflection loss (RL_min) of −63.8 dB. Density functional theory (DFT) calculations and finite element simulations demonstrate that the BIEF effectively modulates charge separation, promotes electron migration, and ultimately improves polarization relaxation loss, leading to superior EWA performance. This study elucidates the intrinsic mechanism by which the FeTe-based heterojunction couples with polarization responses, providing a feasible strategy for the design of lightweight, efficient, and high-performance electromagnetic wave absorbers based on other high-density transition metal telluride (TMT) materials.