Carbon dots (CDs) as versatile carbon-based nanomaterials have attracted increasing attention because of their non-toxicity, good water solubility and photostability, and easy surface functionalization. For their wide application, it is still needed to explore moderate and facile methods for synthesizing CDs from green and inexpensive precursors. In this paper, a moderate method was developed to synthesize water-soluble CDs by ionothermal treatment of cellulose with SO3H-functionalized acidic ionic liquid as a catalyst in 1-butyl-3-methylimidazolium chloride ([Bmim]Cl). The preparation process was carried out at relatively low temperature in non-pressurized vessel. The synthesized CDs exhibit near-spherical morphology with an average diameter of 8.0 nm, and the surface is carbon and oxygen rich. The CDs have powder-blue fluorescence with excitation-dependent emission behavior and excellent stability. Moreover, the as-prepared CDs were demonstrated as an effective "turn-off" fluorescent probe for the selective detection of Hg2+ with a good linear relationship over the concentration range from 6 to 80 mu M. The application of acidic ionic liquid should provide a new path for the synthesis of CDs under mild condition.