As the requirement limitation of toxic contaminants such as heavy metals in food and environmental samples is lower and lower, the detection methods developed should be simpler, sensitive and less contaminated. Luminescent carbon dots nanomaterials are a new type of nanomaterials with carbon as a basic skeleton which have been shown low toxicity and excellent fluorescence performance. In our work, low-cost, eco-friendly carbon dots nanomaterial was prepared by simple hydrothermal method using dopamine hydrochloride as carbon source and o-phenylenediamine as a modifier. The carbon dotsemission wavelength was 596 nm (λ_ex=532 nm). The UV-Vis spectra, fluorescence spectra, infrared spectrum and TEM characterization showed that the carbon dots had good optical performance, good dispersion and uniform size (~3 nm). Under the same conditions, the synthesized carbon dots were found selected detection of Hg²⁺. The detection of mercury is very important, because it is very poisonous and usually presents in the water sample as a form of Hg²⁺ions, and has even irreversible effect on the human being from the contaminated of bio-products in the water. The Stern-Volmer plot with an upward curvature indicates a bindinginduced quenchingof carbon dots at the presence of Hg²⁺ ions, andthe range of 0~0.04 μg·mL^-1(R²=0.992 4) and 0.6~10 μg·mL^-1 (R²=0.993 2) showed good linear correlation. The detection limit was achieved as 0.000 6 μg·mL^-1 (S/N=3, n=6) which meet the Chinese guidelines for both drinking water and industrial sewage. The spiked recovery of Hg²⁺ detection in domestic water samples ranged from 91% to 106% (RSD=0.5 to 1.5). In addition, the biosafety of the prepared carbon dots nanomaterial was further investigated, the results of the MTT for cytotoxicity testing in liver cells showed no significant decrease in cell viability by increasingthe concentration of carbon dotsto 40 μg·mL^-1, indicating that the carbon dots nanomaterial were low toxic and eco-friendly. The high-performance near-infrared carbon dots nanomaterial have good potential in the application of environmental monitoring and biosensor.