Fluorescence imaging in the second near-infrared window (NIR-II, 1000-1400 nm) is attracting extensive attention. Mn6+ doped BaSO4 with broadband emission from 900 nm to 1400 nm is emerging as a new class of NIR phosphor for fluorescence imaging. Manganese has diverse valence states, thus it is difficult to prevent valence change of Mn6+ during traditional synthesis process. In this work, BaSO4:Mn6+ nanoparticles with uniform size and morphology were first successfully prepared through a fast liquid-solid solution route at room temperature. The nanoparticles exhibit broadband NIR emission from Mn6+ when excited by 808 nm lasers. This convenient strategy, based on an efficient anion exchange reaction, is proved effective for synthesizing nano-sized materials. The results reveal that our strategy has great potential in fabricating special valence state ion doped nanomaterials.