The pollution of Hg(Ⅱ) in water environment is extremely harmful to the ecological environment and human health. The current methods for Hg(Ⅱ) detection mainly include atomic spectrometry, mass spectrometry, electrochemistry and so on. But the traditional detection methods need expensive equipments, complicated operation processes, and complex sample preparation, which limit the applications of these methods for Hg(Ⅱ) detection in real samples. It is still a great challenge to develop a sensitive, rapid, simple and cost-effective method for trace Hg (Ⅱ) detection in aqueous solutions. Test paper method is a rapid detection method which transfers the chemical reactions from the glass instruments to test paper. Based on the chemical reaction between the reagents and the targets, the test paper can qualitatively or semi-quantitatively detect the targets through color changes. Carbon dots (CDs), which are the carbon-based nanomaterials with particle sizes less than 10 nm, have many excellent fluorescence properties including low toxicity and high chemical stability. Inspired by the test paper method, a two-color fluorescent test strip for Hg(Ⅱ) detection in water was constructed based on the fact that the fluorescent of CDs could be effectively quenched by Hg(Ⅱ). The sensor, comprised of nitrogen doped carbon dots (NCDs) and rhodamine B (RhB), exhibited dual color emissions at 437 and 575 nm respectively under a single excitation wavelength of 350 nm. When the detection system is added with different concentrations of Hg(Ⅱ), the photoluminescence of the NCDs can be quenched by Hg(Ⅱ) due to synergetic strong electrostatic interaction and metal-ligand coordination between the surface functional group of NCDs and Hg(Ⅱ), while the fluorescence of RhB remains unchanged, and Hg(Ⅱ) can be quantitatively detected based on the ratios of the dual fluorescence emissions (F440/F580). Under the optimized detection conditions of 1 mmol·L-1 HAc-NaAc buffer solution at pH 7, the ratios of F440/F580 were linearly corresponded to Hg(Ⅱ) at the concentrations ranging from 0 to 3 μmol·L-1 with a linear equation of F440/F580=-0.785 2cHg (Ⅱ)+3.103 8 (r>0.99). The detection limit was 2. 7 nmol·L-1 (n=9) based on three standard deviations. The adding standard recoveries of Hg(Ⅱ) detection in lake water and tap water ranged from 91.9% to 117.9%. A visualized two-color fluorescent test strip was prepared by a simple soaking method of NCDs and RhB under optimal conditions. Upon the addition of different concentrations of Hg(Ⅱ), the color of test paper changed from light purple to orange accordingly under a UV lamp (365 nm), in which each of the detection time took only 3 minutes, and Hg(Ⅱ) could be detected as low as 10 nmol·L-1 by naked eyes. In addition, the detection method presented excellent specificity. Therefore, the established method has the advantages of high sensitivity and accuracy, easy operation and portability, and can be used to rapidly detect Hg(Ⅱ) on-site in water environment.