Abstract: To enhance the anti-parasitic efficacy of the natural compound curcumin against Ichthyophthirius multifiliis, this study developed a curcumin-loaded penetration-enhanced nanoemulsion (Cur-CNEs) and evaluated its parasiticidal activityand safety. The formulation components were optimized through solubility and emulsification screening, identifying oleic acid as the oil phase, polyoxyl 40hydrogenated castor oil (RH40) as the emulsifier, absolute ethanol as the co-emulsifier, and 5-fluorouracil as the penetration enhancer. Cur-CNEs were prepared using an ultrasonic emulsification method. The resulting nanoemulsion exhibited a particle size of 6.6±0.29 nm and a polydispersity index (PDI) of 0.074±0.022. Transmission electron microscopy confirmed its uniform droplet morphology and distribution. AO-PI fluorescent staining demonstrated that Cur-CNEs effectively compromised the membrane integrity of I. multifiliis theronts, thereby facilitating drug penetration. In vitro parasiticidal assays revealed that the median effective concentration (EC₅₀) of Cur-CNEs was 0.96 mg/L. A 100% mortality rate was achieved within 4h at a concentration of 2 mg/L, which was significantly more potent than curcumin solution (requiring 8 mg/L). In vivo tests on infected goldfish showed that 78% of the parasiteson the body surface were eliminated after treatment with 2 mg/L Cur-CNEs. Safety assessments indicated a 96h median lethal concentration (LC₅₀) of 8.12 mg/L for goldfish. In a 21-day chronic toxicity study, survival rates remained 100% at concentrations of 2, 2.5, and 4 mg/L. Histopathological examination of liver tissue and analysis of blood biochemical parameters showed no significant alterations compared with the control group across all tested concentrations. In conclusion, Cur-CNEs significantly enhance the efficacy of curcumin against I. multifiliis while exhibiting a favorable safety profile in fish at therapeutic concentrations. This study provides a promising new strategy for the green control of ichthyophthiriasis in aquaculture.