Room‐temperature phosphorescence (RTP) from polymer carbon dots (PCDs) has attracted great attention for its potential applications in manufacturing anticounterfeit objects and digital encryption. However, most RTP raw materials with good performance are generally toxic, corrosive, and polluting, which will cause safety hazards during use and subsequent processing. Herein, facile, environmental‐friendly polymer carbon dots are prepared by hydrothermal treatment of gelatin and carboxylate (C‐PCDs) for the first time. The RTP of C‐PCDs is yellow emission and the lifetime up to 422 ms, which is three times that of PCDs produced only by gelatin. Then according to transmission electron microscope, atomic force microscopy, Fourier transform infrared, X‐ray photoelectron spectroscopy, gel permeation chromatography characterizations, and comparison of optical properties, it is found that the newly CONH formed by introduced carboxylate and free amino produced in the hydrolysis has an inhibitory effect on the hydrolysis of amide bonds contained in gelatin and deamination to a certain extent, which will be thought about more n‐π* transition from CO enhancing the conversion for triplet excitons. Moreover, the C‐PCDs with properties of fluorescence and phosphorescence are confirmed in the potential application of double anticounterfeiting under the excitation of ultraviolet lamps.