Traditional microwave absorbers usually use the epoxy resin dielectric board, which is neither flexible nor optically transparent, so cannot be used in certain applications. In this work, a flexible and optical-transparent broadband absorber is reported, which is based on the indium tin oxide (ITO) and polyethylene terephthalate (PET) material system with an ITO-PET-ITO structure. It showed more than 75% transmittance in the visible light range. The microwave absorption was above 0.8 from 19.9 GHz to 51.8 GHz. The absorption decreased when the absorber was bent, though the variation was not big. Further analysis pointed out that the absorption decrease could be attributed to the change of the incident angle. Theoretical simulation results agreed well with the measured absorption spectra.