Owing to its high electrical conductivity, lightweight nature, and tunable surface properties, MXene has promising application prospects in the field of electromagnetic wave absorption. However, the practical application performance of MXene in electromagnetic wave absorption is significantly restricted by nanosheet agglomeration, insufficient impedance matching, and relatively single loss mechanisms, which markedly limits the full utilization of its intrinsic electromagnetic response in practical absorber design. In this work, Nb₂CT_x MXene and hexagonal boron nitride nanosheets (BNNS) were selected to construct a composite microwave absorbing system. The Nb₂CT_x/BNNS composites with different BNNS contents were prepared by ball milling method. Their microstructure, electromagnetic parameters, and microwave absorption properties were systematically investigated and comparatively analyzed. The results show that ball milling treatment can reduce the lateral size of BNNS, induce edge curling, and decrease the nanosheet thickness, thereby facilitating their subsequent combination with Nb₂CT_x. The composites retain a two-dimensional layered morphology, and the introduction of an appropriate amount of BNNS not only modifies the interlayer stacking structure and lamellar arrangement state of the composites but also promotes interfacial chemical coupling and interfacial charge transfer between the two components. The strong mechanical force of ball milling triggers mechanochemical reactions, inducing the formation of local Nb-N chemical bonding, and introducing defects and residual functional groups onto the surface of Nb₂CT_x, which enhances the interfacial polarization and dipole polarization relaxation of the composite material. When the BNNS content is 10%, the Nb₂CT_x/BNNS composite achieves a relatively optimal balance between impedance matching and attenuation capability, delivering a minimum reflection loss of -68.5 dB at 6.61 GHz in the C band with a matching thickness of 3.312 mm, and an effective absorption bandwidth of 1.53 GHz. The results indicate that the appropriate introduction of BNNS can significantly improve the microwave absorption capability of Nb₂CT_x based composites by suppressing MXene agglomeration, constructing a reasonable conductive network, and enhancing interfacial polarization effects. This endows the composites with promising application potential in C-band electromagnetic wave absorption.