In this paper, we reported a new and environment-friendly strategy to exfoliate graphitic carbon nitride (CN) in hot water to obtain ultrathin CN nanosheets (CNNS). By thermal treating, water molecules were intercalated into the interlayer space of bulky CN and further hydrolyzed the bridge-linked N groups between tri-s-triazine units, thereby cutting the CN layers into CNNS. Due to the negative charges on surface (Zeta potential was -13 mV at pH 7), the CNNS colloids were extremely stable. The physicochemical characterization indicated that the as-prepared CNNS had a typical 2D morphology with a 1.2 nm thickness and numerous -OH groups on surface. Moreover, the high charge separation and transport ability were achieved in CNNS because of the retaining of conjugated CN system. Compared to the bulk CN, the as-prepared ultrathin CNNS exhibited an enhanced photocatalytic performance (four times higher than that of the bulk CN) for degradation of organic dye under visible light irradiation. Additionally, the superior reusability and the excellent generality of CNNS for decomposing other pollutants were also demonstrated. Finally, we proposed a possible mechanism of CNNS based on the examined band structure and the main active species determined by quenching of various active species.