Abstract: Traditional microwave anechoic chambers suffer from high cost, large building area and immovability, which limits their applications in practical engineering scenarios. Researches about wavelength-scale miniaturized anechoic chambers is crucial for antenna design and performance analysis in fields such as communication and microwave systems. Based on passive artificial media, this paper constructs a conformal and dual-frequency omnidirectional reflectionless absorbing surface with a 0.636 mm thick which can work at 5.1 GHz and 9.0 GHz simultaneously. Furthermore, a semi-closed cylindrical chamber with a diameter of 34.38 cm is constructed based on COMSOL Multiphysics. Full-wave simulation verified the effectiveness of the constructed dual-frequency absorbing units. The spatial voltage standing wave ratios inside the anechoic chamber calculated at two frequencies are 1.018 and 1.010, respectively, which meet the requirements of industrial experiments. This proposed miniaturized anechoic chamber work at dual-frequency, and has the advantages of portability, simple structure and good performance, and is expected to be widely used in practical engineering scenarios.