Optical modulation is significant and ubiquitous to telecommunication technologies, smart windows, and military devices. However, due to the limited tunability of traditional doping, it is a tough problem to achieve broadband optical property change. Here, we demonstrate a remarkable transformation of optical transmittance in few-layer graphene (FLG) covering the electromagnetic spectrum from the visible to the terahertz wave after lithium (Li) intercalation. It results in that the transmittance is higher than 90% from the wavelength of 480 to 1040 nm and increases most from 86.4% to 94.1% at 600 nm, reduces from ~80% to ~68% in the wavelength range from 2.5 to 11 μm, and has ~20% reduction over a wavelength range from 0.4 THz to 1.2 THz and reduces from 97.2% to 68.2% at the wavelength of 1.2 THz. The optical modification of lithiated FLG is attributed to the increase of Fermi energy (Ef) due to the charge transfer from Li to graphene layers. Our results may provide a new strategy for the design of broadband optical modulation devices.