The sufficient interface contact in the composite absorbing material is beneficial to increase the dielectric loss and promote the microwave absorption performance. In this paper, the composite nanoparticles (NPs), FeO covered with ultra-thin carbon layer (FeO/C), were synthesized by simple high temperature solution-phase and subsequent high-temperature steam carbonization methods. Small size FeO/C composite NPs have large heterogeneous interfaces, which can control the polarization loss of composite NPs through the method of interface regulation and achieve high microwave absorption performance. The strongest reflection loss of the composite NPs with an average particle size of 52 nm can reach -58.5 dB at 14.88 GHz with a thickness of 2 mm, and the corresponding effective absorption (RL ≤ -10 dB) bandwidth (EAB) is 5.63 GHz (12.37-18 GHz). In particular, the high-efficiency absorption (RL ≤ -20 dB) bandwidth of FeO/C can reach 15.44 GHz (2-17.44 GHz) with a thickness of 1.7-10 mm. The current method for controlling polarization loss provide a meaningful reference for future microwave absorption research.