Pistol-like DNAzyme (PLDz) is an oxidative DNA-cleaving catalytic DNA with ascorbic acid as cofactor. Herein, glutathione was induced into the reaction system to maintain reduced ascorbic acid levels for higher efficient cleavage. However, data indicated that glutathione played triple roles in PLDz-catalyzed reactions. Glutathione alone had no effect on PLDz, and showed inhibitory effect on ascorbic acid-induced PLDz catalysis, but exhibited stimulating effect on Cu2+-promoted self-cleavage of PLDz. Further analysis of the effect of glutathione/Cu2+ on PLDz indicated that H2O2 played a key role in PLDz catalysis. Finally, we developed a fluorescent Cu2+ sensor (PL-Cu 1.0) based on the relationship between glutathione/Cu2+ and catalytic activity of PLDz. The fluorescent intensity showed a linear response toward the logarithm concentration of Cu2+ over the range from 80 nM to 30 μM, with a detection limit of 21.1 nM. PL-Cu 1.0 provided only detection of Cu2+ over other divalent metal ions. Ca2+ and Mg2+ could not interfere with Cu2+ detection even at a 1000-fold concentration. We further applied PL-Cu 1.0 for Cu2+ detection in tap and bottled water. Water stored in copper taps overnight had relatively high Cu2+ concentrations, with a maximum 22.3 μM. Trace Cu2+ (52.2 nM) in deep spring was detected among the tested bottled water. Therefore, PL-Cu 1.0 is feasible to detect Cu2+ in drinking water, with a practical application.