The aim of this study is to analyze the structural characteristic and fitness cost of class II integron， a drug-resistant mobile genetic element of Escherichia coli from aquatic food. Each 200 samples of commercially available Pacific white shrimp， oyster and large yellow croaker were collected from Zhejiang Province， and E. coli were isolated and identified for analyzing their drug-resistance characteristics and class II integron carriage rates. Then the fitness cost of E. coli carrying class II integrons in a non-antibiotic resistant environment and at sub-inhibitory concentrations of antibiotics were compared using competition tests and flow cytometry. The results showed that 28 multidrug-resistant strains of the 278 E. coli strains isolated carried class II integrons， all of which contained functionally deficient integrase structures. Of these， 27 contained typical drug-resistance gene cassettes dfrA1-sat2-aadA1 and one contained an atypical gene cassette dfrA1-catB2-sat2-aadA1. Under the non-antibiotic condition of LB culture， the selection coefficients S for the above 2 types of class II integrons were -0.068 and -0.117， respectively， both less than 0， indicating that there is a fitness cost for E. coli carrying class II integrons in a non-antibiotic environment. At sub-inhibitory antibiotic concentrations (40% MIC and 20% MIC)， the selection coefficients S for the above 2 types of class II integrons were greater than 0 and were well adapted. In conclusion， class II integrons are already present in a certain proportion of E. coli from aquatic food. In addition， low concentration of antibiotics is conducive to the maintenance and enrichment of class II integrons， so as to promote the effective response to external antibiotic pressure and spread among bacteria.