Abstract: Since traditional per- and polyfluoroalkyl substances (PFAS) threaten the ecological environment and human health, the ultrashort-chain perfluorinated compound trifluoromethanesulfonic acid (TFOH) has become a mainstream alternative, and its potential risks to aquatic ecosystems require urgent assessment. This study used Daphnia magna as the model organism to systematically investigate the toxic effects and mechanisms of TFOH. Acute toxicity experiments showed that the 24 h-LC₅₀ and 48 h-LC₅₀ of TFOH for D. magna were 27.59 mg/L and 12.81 mg/L, respectively, classifying it as a low-toxicity substance. Chronic experiments revealed that TFOH has a dual concentration effect on the growth and reproduction of D. magna: low concentrations (300 ng/L～3 μg/L) promoted reproduction, increasing offspring number and population growth parameters; high concentration (30 μg/L) inhibited growth, molting, and other physiological activities. Regarding the mechanism, TFOH can induce oxidative stress and neurotoxicity, manifested as superoxide dismutase (SOD) activity first increasing and then decreasing, and acetylcholinesterase (AChE) activity showing a nonlinear change of "low-dose inhibition and high-dose promotion", thereby interfering with neural transmission and behavioral functions. The results of this study provide key toxicological evidence for the ecological risk assessment of TFOH.