Abstract Utilizing the flexible spatial structure and dynamic conformational change of DNA tweezer (DT), a simple and fast fluorescent aptasensor based on fluorescence resonance energy transfer (FRET) was designed. The hinge region and the overhang region were optimized to obtain the satisfied flexibility and controllably conformational change. Using ochratoxin A (OTA) as a simulation target, the established aptasensor showed a simple and time‐saving procedure, higher sensitivity, good selectivity, and good stability with a detection limit of 0.024 ng/ml. Besides, the aptasensor exhibited favorable accuracy with recoveries of 95–103% for spiked cereal samples. The proposed fluorescent aptasensor has a promising potential for the rapid and reliable detection of targets.