By the method of high-performance liquid chromatography (HPLC), this paper studied the hydrolysis characteristics of enrofloxacin under conditions of different pH, light, and microorganism to assess its ecological risk. The results showed that no ciprofloxacin was found in the hydrolyzed products of enrofloxacin. Under the conditions of 50 degrees C and pH 1 - 10, the hydrolyzation rate of enrofloxacin was less than 10% within 5 days. The half-life of enrofloxacin was longer than one year at normal temperature and in dark, and buffer's pH had no significant effects on the hydrolysis of enrofloxacin. The degradation of enrofloxacin in natural water was strongly affected by light. Under outdoor natural light condition, enrofloxacin degraded quickly and was not examined after 3 days; while under indoor natural light condition, it degraded slowly, with the degradation rate being 48%, 72% and 65% after 31 days when its initial concentration in water was 0.05, 0.2 and 1.0 mg x L(-1), respectively. Enrofloxacin was very steady in dark, and microorganisms had no significant effects on its degradation. Owing its rapid degradation under outdoor natural light condition, enrofloxacin might have no direct harmfulness to aquatic ecosystem, but its potential ecological risk should not be neglected once entering into aquatic ecosystem, especially in bottom mud.