Phosphorescence processes in composite systems based on luminescent carbon dots (CDs) are of great fundamental and practical interest. Herein, the control of the phosphorescent signal in a CD/polyvinyl alcohol (PVA) composite is realized. Enhanced green phosphorescence is obtained via thermal annealing of the composite under 200 °C, which can be quenched via water treatment, and then recovered again, via either repeated annealing or near-infrared laser irradiation. Water molecules infiltrating the CD/PVA composite lead to considerable vibrational motions of the CD surface groups through destruction of the rigid composite structure together with an increase in the possibility of the penetration of the phosphorescence quenchers, and thus a decrease in the phosphorescence intensity. After repeated annealing, the nonradiative transitions are restricted, and the phosphorescence signal is recovered again. Based on such on-off switching of the phosphorescence signal, a concept of multiple data encryption is realized by using the CD@PVA composite.