DNAzymes are promising agents for theranostics and biosensors. Sodium dependent DNAzymes have been developed for sensing and imaging of Na+, but these DNAzymes have low catalytic activity. Herein, we demonstrate that a molecular crowded environment containing 10 to 40 wt% PEG enhanced the catalytic activity of a Na+-dependent DNAzyme, EtNa, although dextran did not. The cationic copolymer poly(L-lysine)-graft-poly(ethylene glycol) at 0.03 wt% (0.3 g L-1) enhanced the reaction rate of EtNa by 10-fold, which is similar to the acceleration induced by 15 wt% (150 g L-1) PEG. A cooperative impact of the copolymer and crowding agent was observed: the combination resulted in an impressive 46-fold acceleration effect. Thus, the use of a cationic copolymer and a crowding agent is a promising strategy to improve the activity of Na+-dependent DNAzyme-based nanomachines, biosensors, and theranostics, especially in environments lacking divalent metal ions.