The trans-cleavage activity of CRISPR/Cas12a has been widely used in biosensing applications. However, the lack of exploration on the fundamental properties of CRISPR/Cas12a not only discourages further in-depth studies of the CRISPR/Cas12a system but also limits the design space of CRISPR/Cas12a-based applications. Herein, a "RESET" effect (random extending sequences enhance trans-cleavage activity) is discovered for the activation of CRISPR/Cas12a trans-cleavage activity. That is, a single-stranded DNA, which is too short to work as the activator, can efficiently activate CRISPR/Cas12a after being extended a random sequence from its 3'-end, even when the random sequence folds into secondary structures. The finding of the "RESET" effect enriches the CRISPR/Cas12a-based sensing strategies. Based on this effect, two CRISPR/Cas12a-based biosensors are designed for the sensitive and specific detection of two biologically important enzymes.