Abstract: The research was performed for the rational usage of enrofloxacin in treating pigs infected by Escherichia coli. The combined method of pharmacokinetics in vivo and pharmacodynamics in vitro was used to survey and evaluate the pharmacokinetics-pharmacodynamics relationship of enrofloxacin against Escherichia coli. The MICs of enrofloxacin against Escherichia coli in serum or tissue cage fluid were 0.4 μg·mL^-1 in vitro. The concentration of enrofloxacin that is more than MIC in serum or tissue cage fluid will be needed to inhibit the bacteria if lots of Escherichia coli was added. EC₅₀ were 77.67±31.12 and 15.78±4.99, respectively in serum and tissue cage fluid of pigs after intramuscular administration at 5 mg·kg^-1 body weight. Enrofloxacin produces 50% of the maximal antibacterial effect, when the concentration of enrofloxacin is 1.29 and 0.26 μg·mL^-1, respectively in serum and tissue cage fluid. These results indicated that the dosage of enrofloxacin should be increased when it was used to prevent the infection of Escherichia coli, and MIC shouldn’t be the only standard that set interval of administration.