The invention belongs to the technical field of chemical application, in particular relates to a method for preparing a fluorescent carbon spot solution from chloroplast as a carbon source by using ahydrothermal method, and aims to detect pH values. A method for detecting the pH value of a solution comprises the following step: by taking chloroplast as a carbon source, water as a dispersion liquid, and under a heating condition of a hydrothermal reaction kettle, synthesizing a fluorescent carbon spot solution, thereby obtaining a carbon spot solution. The fluorescent carbon spot solution withfluorescence properties is prepared; after the fluorescent carbon spot solution is reacted with solutions of different pH values, fluorescence intensity values of the fluorescent carbon spot solutionand the pH values are in a good linear relationship IF=395.44-10.36x, in the equation, IF is a fluorescence intensity value, x is a pH value, and the detectable pH values of the solutions are within1.83-10.38; the method is simple in process, fluorescent carbon spots are rapidly prepared, and the pH values of solutions are precisely tested.