Water is one of the most important natural resource. However, in some cases, it is imperative to monitor the water content for safety considerations as it is detrimental to the chemical synthesis, drug safety, etc. In this study, palmatine, a natural isoquinoline alkaloid, was proposed as a fluorescent probe for this purpose. Palmatine was demonstrated to be extremely sensitive to water with the obvious fluorescence quenching of it. In light of this phenomenon, the water content in six organic solvents, including acetonitrile, methanol, ethanol, n-propanol, acetone and 1,4-dioxane, could be determined using palmatine as the fluorescent probe, which had wide detection ranges (0-100%) and low detection limits (e.g. 0.08% in ethanol) depending on the individual organic solvents. Moreover, a novel turn-off-(on-off) fluorescent colorimetric probe was constructed based on the combination of palmatine and nitrogen doped carbon dots (N-CDs) (palmatine/N-CDs). The blue fluorescence of N-CDs was quenched by palmatine while the green-yellow fluorescence of palmatine remained; upon interacting with water, the green-yellow fluorescence of palmatine was quenched, while the blue fluorescence of N-CDs recovered. Thus, with the elevated water content, the fluorescent color transitioned from green-yellow to blue, making the visual semi-quantification of water content into a reality. Furthermore, with the assistance of smartphone and APP (Color Acquisition), the quantification of water content could be realized via only photographing. Using palmatine as the fluorescent probe, or palmatine/N-CDs as the fluorescent colorimetric probe, water contents in disinfection alcohol and nitroglycerin injection were accurately determined, and both results agreed well with those of the classical Karl Fischer method. Palmatine was demonstrated to be a potential fluorescent probe for sensing water, and the novel turn-off-(on-off) fluorescent colorimetric probe based on it was promising for the on-site fast detection of water.