In this study, a broadband radar absorbing sandwich composite (BRASC) with stable absorption performance for oblique incidence is proposed to reduce radar cross-section (RCS) of an engine inlet. To design the BRASC, the objective function for broadband absorption performance is coupled with transmission line theory. The absorption bandwidth of the designed BRASC with a − 10 dB criterion is 8.2 GHz. The absorption performance of the oblique incidence in both transverse electric and transverse magnetic polarizations is excellent in maintaining the −10 dB up to 60°. The measurement results of the fabricated specimens confirm that the overall tendency of the measurement results matches well with the analysis results. The engine duct replaced with the BRASC is analyzed to confirm the effect of RCS reduction on the engine inlet. RCS simulation shows that the proposed BRASC effectively reduces the RCS over a wide frequency range. Because the S-shaped duct, which is used in low observable aircraft for RCS reduction, causes multiple scattering at oblique incidence, the absorbers applied to the engine duct should maintain a stable oblique incidence performance. Hence, the proposed BRASC may be a promising candidate for engine inlets with small RCS.