Due to the high concentration of I^- and Br^- content in the sea water, the electrolysis process for ballast water will produce a certain content of iodo-trihalomethanes (I-THMs), which has the strong carcinogenic and mutagenic properties. This study is to investigate the mechanism of forming the disinfection by-products, iodo-trihalomethanes, and the influence factors by dosing sodium hypochlide.The results showed that:(1) when NaClO concentration increased from 5mg/L to 15mg/L, the concentration of I-THMs increased by 24.76 g/L; (2) the amount of I-THMs increased with the increasing of temperature and when the temperature is higher than 20 DEG C, I-THMs was generated rapidly; (3) under acidic condition, the production of I-THMs generated is small while under alkaline conditions, the production of I-THMs increased significantly; (4) for the solution with different ratios of bromine and iodine, I-THMs was generated with the different proportions of each component. Therefore, it is suggested that the best reaction conditions should be optimized in the process of ballast water electrolysis in order to ensure the treatment efficacy and effectively control the generation of disinfection by-products DBPs.