A bio probe was firstly prepared by assembling sulfhydryl DNA on the surface of Au nanoparticles(AuNPs).Then the 3′-OH end on the probe DNA was extended under the catalysis of terminal deoxynucleotidyl transferase(TdTase),and produced a long single chain DNA(single stranded DNA,ssDNA).The probe and the long ssDNA were characterized by agarose gel electrophoresis and atomic force microscopy(AFM).Results showed that,under the action of TdTase,DNA containing 3′-OH achieved an efficient extension at the nano interface,and the chain length was up to 850 nm though it was in the irregular coiled state.Afterwards,this DNA growth strategy combined with aptamer technology was applied to construct an aptasensor for the detection of enrofloxacin(ENR).The aptamer fragment 1 was firstly assembled on the surface of the AuNPs,then the aptamer fragment 2 was assembled on the surface of the magnetic particle activated by glutaraldehyde.After experiencing the TdT-mediated DNA growth strategy,the aptasensor had a good response to ENR concentration in the range of 10-10×105 ng/mL with a detection limit of 10 ng/mL.At the same time,the detection signal for enrofloxacin was significantly different from those for ciprofloxacin,ofloxacin,norfloxacin and other control groups.The results showed that the TdTase based aptasensor could provide a new research idea or method for food safety detection and other fields with the advantages of convenient operation,good sensitivity and excellent specificity.