Objective To establish a label-free aptasensor based on the fluorescent dye thiazole orange (TO) and aptamer 45e-1 for rapid and sensitive detection of saxitoxin (STX). Methods The aptamer 45e-1 (50 nmol/L) was dissolved in buffer (20 mmol/L trihydroxymethyl aminomethane-HCl, 100 mmol/L NaCl, 2.5 mmol/L MgCl₂, 5 mmol/L KCl, pH=7.5). It was then heated in a water bath at 95℃ for 10 min, followed by cooling in an ice-water bath for 5 min. The STX standard solution was added, thoroughly mixed, and then incubated at room temperature for 10 min. After that, TO solution was added, and buffer was also added to make the reaction system volume 100 μL. The mixture was thoroughly mixed and incubated for 2 min at room temperature. Finally, the fluorescence intensity of the system was measured at excitation wavelength of 496 nm. Results When the molar concentration ratio of TO to 45e-1 was 5:1, the incubation time of TO and 45e-1 was 2 min, the incubation time of STX and 45e-1 was 10 min, the concentration of Mg²⁺ was 2.5 mmol/L, and the concentration of K⁺ was 5 mmol/L, the changes in fluorescence intensity showed a good linear relationship with STX concentration. The fitting linear equation was Y=3 639.8X+2 341.5 (R²=0.972 5; X was the common logarithm of STX concentration[nmol/L]), and the detection limit was 0.67 nmol/L. The cross-reaction with other common marine toxins was negligible. In seawater, the recovery rate was 90.7 %-108.7 %, and the relative standard deviation was 7.1 %-10.9 %. Conculsion The label-free aptasensor established in this study can achieve the quantitative detection of STX with good specificity and reproducibility.