5G mobile communication is developing at a faster-than-expected pace, especially in the mid-frequency band (so-called sub-6G band). This paper presents a transparent and flexible broadband absorber for the sub-6G band of 5G mobile communication. A sandwich-structured metamaterial absorber (MMA) was designed and tested. The transparent and conductive material Indium Tin Oxide (ITO) was used for the surface resonant structures and the backplane ground layer; and polyethylene terephthalate (PET) and polydimethylsiloxane (PDMS) were used as the dielectric layer, separately, both are transparent and flexible. The absorber featured >80% broadband absorption, covering a wide frequency range of (3.0~10.0) GHz for PET dielectric layer, and (3.2~11.0) GHz for PET-PDMS-PET dielectric layer. The thickness of the absorber made of the latter was 6.25 mm only (0.067 times of the wavelength corresponding to the lowest absorption frequency). With additional advantages of excellent flexibility and transparency, the MMA perfectly covers the frequency bands of the sub-6G band and can play an active role in the 5G communication in the near future.